Man it all depends on the situation. If you know that you're going to see this girl often, it's in your best interest to make sure that she's interested before you head off and approach.
It could be good, but you don't want any awkwardness between you guys. Just say hi back and "make your moves" if she's interested.
If you don't see her often, go ahead and approach, you have nothing to lose. Even if the approach doesn't go well, you're never gonna have to see her again so don't worry about it!