Thai people do not expect foreigners to do the wai but it is a respectful and courteous ting to do in different situations. For example, if someone wai’s to you first then you are expected to wai back, unless the person is a child or someone serving you, such as a maid. However, there is no harm done if you do wai to these people. You can also wai to people who you are meeting for the first time or when you are greeting people generally.
The higher you hold your hands and the longer you bow you head shows that you are being even more respectful. For instance, this is appropriate if you are meeting your girlfriend’s parents for the first time.