I agree, I don't like to tip for blowjobs either. I also prefer to pay by cash for those.

Seriously though, I don't think I tip based on attractiveness. But I do tend to tip more as I get more drunk. A few of my friends even check how much I'm tipping if they think I'm too drunk and tell me to lower it. They've probably saved me around $100 in the last few months lol.