...when you check out their bodies before you start up a conversation? It seems to me like girls immediately think I'm ungentlemanly and depraved if I look at them a little too lustfully. Thoughts?

Mainly yes.

Try look no lower then her face.

okay, that being said, are there times where it can work in you favour?

I can't really think of a situation where it helps you, only hurts. Stop doing it.