I am a firm believer in tipping for good service. However, I am smart enough to know what is expected in the country I am visiting.
In the United States, a tip of 15% is the norm. And in many places, it is added automatically. I understand that the server has to report 8% to the Internal Revenue Service regardless of how much they receive. So, if the service is good, 15% is acceptable.
In Thailand, this is not normal. Unless you are at a 5-star hotel or restaurant, you should only tip 5 or 10 percent and only if the service is exceptional.
You will find in many bars that your change will be placed in a small booklet or on a tray and they will always figure out the change to leave some very small coins on the tray. If your change is 400 Baht, you can expect 300 Baht in 100 Baht notes and the rest in smaller money. They will expect you to leave them a tip.
Some will even hide the coins under the bills expecting you to just grab the paper money and leave the coins. This annoys me to no end.
There are some that automatically expect a tip even if the service is crappy. And you will get dirty looks if you take all of your change and pocket...