I'm definitely more ticklish if I don't see it coming. It's always great when a lady comes up behind you and tickles your ribs, or your sides to get your attention. That's my favorite way to be approched. It tells you that she thinks you're cute enough to tickle, that she thought about it, and she might even want to just find out if you're ticklish. It's fun to see the expression on her face when you jump, too! It's usually a sly little smile, as if to say "ha, ha, you're ticklish..."
My ex-girlfriend surprised the heck out of me the first time she tickled me. She tickled me pretty hard, made me laugh, then yelled to her brother and his date, "Hey, he's ticklish!" Man, did she ever pay for that one!