As well as being a greeting, a wai is a show of respect. It can be difficult to determine when you should wai or when someone should wai you. As a foreigner, it's just easiest to wai important people as soon as possible. If someone is going to wai you first, they probably already beat you to it.
However, if you really want to know when to wai, this depends on your relationship to the other person. The person of higher social status will be wai-ed to. You have only a few seconds to size up someone you meet and determine this, but it's best to err on the side of waiing first if you are unsure.