It depends...
A lot of places pool tips and then equally divide them. That makes me mad.
If I get crappy service, my waitperson splits the tips of the hard workers and more deserving people. So, where I know the server actually keeps the specific tip, I tend to tip better for better service.
For places that pool, I tend to be cheaper.
If I get a good haircut, and the person was generally a decent worker and actually cared how my hair turned out- a better tip.
I've had to get up and get my own condiments from a different table, go to the waitress station and get my own extra napkins and so on- that usually happens where they pool tips.
To Insure Promptness