I travel to Thailand and Asia all the time and i have never had a serious problem with any of the customs officers, although give an Asian person a official looking uniform and they do seem to get a bit over officious ( i once got hit with a billy club for repeatedly standing over the white line at a sky train syation) Always smile and be friendly, learn how to say hello and thank you in Thai and that always shows you are at least making a bit if an effort.
Saying that i once got into quite a serious situation when i over stayed a visa in Thailand, but i'm sure that wont apply to you.
The first time i went to Oz i had some funny ***** customs guy who was originaly from Soton, thought it would be funny to give me a bit of a grilling.
Another time i made a bit of a joke with one of the customs guy, he didn't find it funny and spent the next 45 minutes pulling my backpack apart.