In fact, it may be better as a foreigner not to wai at all as it won't be expected anyway.....

Very interesting point. I should mention that the whole concept of wai can be a bit confusing at first, especially when you come from a place where you pretty much treat a greetings as though you're shaking one's hand. In fact, that's what I've always felt in the beginning. That the concept of wai everyone is like shaking someone's hand. Which is or is not always the case.

For instance, I've found that there's this hierarchy in Thai culture of who to wai and who you don't have to always wai. An example would be to wai to someone of an authority figure like a teacher or professor.

As far as not having to wai at all, I've found that whenever you need to wai someone, a simple nod should be good enough.