This is a really nasty habit, so don't do it.
Imagine that someone gave you something and it had their saliva all over it. Well, it is the same thing as you giving an agent your travel documents that were just in your mouth. Especially when you take it straight from your mouth and hand it to them. Not that you should not let them see it in your mouth mind you.
So keep your documents in your pocket, purse, travel bag, anywhere else but in your mouth.
It is funny when you see it happen, but it is really quite disgusting.