Traffic was light when we set off from London to Heathrow Airport early on Christmas Eve.

The minicab got us there in 30 minutes.

While H and the cab driver took the luggage out from the vehicle, I went to get a trolley. When I returned, H was handing over the money to the driver. He'd charged us a flat fee of £70.

You didn't tip the driver, did you, I asked when the minicab drove off.

No, H said.

But I had told you earlier to tip, I protested.

You told me to round up the bill, he said. There was nothing to round up to. It was a short trip and £70 is already a lot of money. Besides, he added, the driver doesn't need it. He probably owns a few taxis.

(During the ride, the driver was on the phone with other drivers arranging pick-ups; he'd also mentioned he was going on holiday soon to Singapore and Vietnam.)

Whether or not he has money is beside the point, I muttered. We should have given him a tip.

I wasn't happy but decided to hold my peace.

I didn't want to spoil the day, especially as it was Christmas. Also, it wasn't like the service had been exceptional, and H had handled most of the luggage himself.

I suppose £70 wasn't a bad sum for an easy 30-minute drive. We also didn't have anything smaller than £5 notes and if we'd given him £75, it would have been too much.

In any case, it was silly of me to be mad with H on account of a stranger.

But the matter preyed on my mind all the way back to Singapore.

To tip or not to tip and how much? This ranks high on the list of what H and I argue about when we go on holiday (No. 1 would be directions, namely why does he find it so hard to ask someone for directions? Why does he prefer squinting at a map?)

The first time we went on holiday as a couple (our Bali honeymoon), he raised his eyebrows at how I was tipping left, right and centre - driver, room service, concierge, porter, stall holders in the markets - and the amounts I gave.

It wasn't that my tips were overly generous, but they were clearly more than what he was used to giving. I suspect he also found my tipping crass and show-offy.

I, on the other hand, was disturbed by how tipping wasn't always at the forefront of his mind and that he had to be reminded to do so. Had I married a tightwad?

Over time, we have come to the conclusion that it's best I decide who and how much to tip, as far as possible.

Problem is, I am not sure myself and can overdo things.

In 2014, we went to France. It was evening by the time we reached the airport and from there it was a two-and-a-half-hour journey to a mountain resort, travelling on narrow roads with sharp bends and with most of the driving done in pitch blackness.

By the time we reached the hotel, I was so relieved that we had arrived in one piece that I gave the driver a €30 tip.

A week later, we drove back to the airport. I gave that driver €30 too, even though he had spent half the trip on the phone arranging pick-ups and frightening me to no end because he kept taking his eyes off the road to make calls.

In retrospect, the tips were excessive.

Last month, when we went to France again, I decided to rein myself in. I gave a €20 tip for the ride to the mountain. On the way back, I handed over €10 because we were sharing the van with a family.

There are two reasons I get frazzled about tipping when I travel.

One, I am unfamiliar with tipping.

Two, I suffer from traveller's guilt.

Singapore does not have a tipping culture. At restaurants, you pay a service charge as well as the Goods and Services Tax and most people I know don't tip on top of all that.

(Interestingly, the Singapore Tourism Board tells tourists on its YourSingapore website that "tipping is encouraged when you experience good service, and this would be above the standard 10 per cent gratuity to your bill added by some restaurants and hotels".)

I prefer a service charge to tipping. Life is much simpler when I don't have to be constantly monitoring the service I am getting and calculating how much it is worth. I'd much rather spend my energy enjoying the meal.

Even if the service at a restaurant isn't great, I don't begrudge the charge because in the bigger scheme of things, it will help the restaurant survive.

Because tips are not expected in Singapore, I seldom tip.

I almost never do so at restaurants. I tip my masseuse, though, because she does a good job, and fast food deliverymen because they lead dangerous lives on the road, and car valets.

I don't tip cabbies except when I'm taking a taxi to and from Changi airport.

Once I leave Singapore, though, I morph into a conscientious tipper.

Much of this has to do with how I get travellers' guilt as soon as I touch down on foreign soil.

As a tourist, the people you meet tend to be service staff - taxi drivers, porters, receptionists, room cleaners, waiters, stallholders, buskers, guides.

Here I am with a week or two in front of me doing nothing but enjoying myself, spending money and fretting over First World concerns - does the room have a Nespresso machine - and there they are, working their butts off for people like me.

I feel bad. (I don't get this guilt at home because I am also working most of the time.)

Traveller's guilt hits me whether I am in a developed city like London, where wages and standards of living are high, or in less-developed places like Yangon.

At the ski resort in France last month, I kept feeling sorry for the harried-looking waiters at the restaurant where guests stuffed their faces at every buffet meal.

In London, I felt awkward whenever I walked past the concierges at our hotel. They worked from day to night.

In Hong Kong last year, I was shamefaced to see the doormen when I returned to the hotel laden with shopping bags.

One reason I tip is to assuage this guilt.

Tipping norms vary from country to country, so not only do I struggle with traveller's guilt, I also spend too much of my vacation time trying to figure out if I'm doing the right thing.

I know tipping is regarded as almost an insult in Japan so I'm on safe ground there - I don't tip.

I also know that if you don't tip, and tip big, in the United States, you risk being heckled by waiters. But what about other countries where rules aren't so well-known?

There are other dilemmas:

Is it better to tip on arrival or when you leave? Tipping at the start will assure you of better service during your stay. But what if you regard tipping less as greasing the wheels and more as a gesture of appreciation?

Is it really the done thing to put money under the pillow for housekeeping?

How many times do you tip room service? It's expected when they bring in the food but when they retrieve the trays too?

Why tip just the concierge? What about the helpful receptionist?

And how much does one tip? Coins seem rather demeaning to me, but is something better than nothing?

I don't think I'll ever know all the answers.

What I need to do, though, is to stop being so precious.

The world doesn't revolve around whether or not I leave a tip.

I should just do what feels right at the time, then let it go and relax - which is why I go on holiday anyway.

•Follow Sumiko Tan on Twitter @STsumikotan