That brings up a good point, you should try to understand the culture your visiting,
Just to say more of the same..... from a non-american point of view.
I recently was spending some time for work and holidays in the US. (New York, Minnesota and Michigan.)
I thought it might be an idea to check on the local customs, and I was adviced to tip about 10-15% when the service is good and 15-20% when the service is excellent.
So when entering restaurants, bars, etc I added 20% in my mind and if the service wasn't excellent, I subtracted.
When my american boss was over in the Netherlands, we went for a meal. He paid, he is the boss and the boss pays (company policy).
The service was poor even for Dutch standards. He still wanted to tip the waitress (he doesn't speak the local lingo and missed out on the poor service). We told him not to. He then wanted to tip a little say 5%. We argued and in the end I told him that any tip left behind by him would be taken by me and given to the first person I would meet in the street that looked like they needed the money.
We are not a culture of (big) tippers. And the last thing I wanted was the waitress not getting the signal that the service was poor. (If my boss wasn't there with us, we would have gotten up and left the restaurant and found another place to eat.)
What I personally think it comes down to:
1. Check where you are going what the local customs are.
2. Realise that the whole rewardsystem in that area will most likely be set up around the local customs.
3. Adjust your normal behaviour to the local customs. As far as you are willing to do.
So if tipping 20% is normal for excellent service, be prepared to hand over 20% when you received excellent service. If that is too much than you couldn't afford the trip/activity/meal etc.
Just my 2 bar worth..