I can also count on one hand the number of flights I can remember with empty F seats.
I have found that on mainline flights the FA tend to check the manifest more often. In fact, my last flight the FA doing pre-flight beverages addressed everyone by name while carrying a copy of the manifest with her. I think sneaking into F is a gutsy move and I would bet 9 times out of 10 the FA will catch you.
I was on a DL Connection flight last year STL-DCA on a Saturday morning and there were three of us in F. The FA moved up a group of her friends who were apparently on the plane. The FA was nice, but here friends were obnoxious.