Only if I'm satisfied.
I nearly died when I was planning my wedding and found out that, on top of the outrageous prices, it is expected that we tip a good 15%. Seriously? I tipped the people that did a good job (seamstress, cake baker, hairdresser, limo, DJ) but the people that owned the location and did the catering were terrible. They tried to overcharge me by almost $1000 (and considering we were 20, had two kids, and were paying for it almost entirely on our own that was a LOT) so I didn't tip them at all.
When you consider the cake was $800 +$80 tip, and I could have made it myself if I wasn't so busy for maybe $30, that is a complete ripoff.
Another tip rant: I worked at a restaurant with some waitresses that thought they were entitled to a tip no matter what. One girl was so incredibly rude, she never checked to see how people were doing, and she bitched if she got less than $10 (on a bill of $40). I'm sorry, but I don't even think she deserved $5 most of the time.
Anyway, if I get a haircut and I like it, I tip. If my kids are difficult and they do a reasonably good job, I tip. But if they nick my kid's ear (its happened), or the hair is completely crooked.... no tip. And for restaurants I always tip, and I try to be generous, because I know how crappy of a job it can be sometimes.