I don’t usually tip in Thailand -but there are small family-owned restaurants that I know the owners and sometimes leave the kids that collect the check ten-baht or sometimes fifteen. If I eat with a large group I might tip 50 baht.
Tipping is not necessary in Thailand – but, it’s a nice gesture considering if you tip someone 20 baht that’s 1/10th of what they’re going to make the whole day and a very nice bonus – while it costs you 66cents at the current dollar exchange rate.
Thais rarely tip. You can do as the Thais do in this case – you probably are paying higher prices than Thais do anyway.