To answer your question. Yes, guys are stiffing our girls. Now, I'm assuming that it is none of us gentlemen. But, it is being done.
I've gotten to the point that I don't visit many new places. I pretty much stay with a fairly large group of regular providers. And as trust builds, they are more willing to talk honestly. And, unfortunately, yes, they are getting cheated out of decent tips ( or any tip,for that matter.)
I know there alot more to this discussion. But, yes, it's going on...