I guess there are many cultural variations when it comes to tipping etiquette, but if I ordered a meal at a restaurant and the service was slow / poor etc , I would have no problem withholding a tip. I'd do the same if I was at an event where the music was lousy.
You have a say in the music played, though. When I dj an event, I want people to request, mostly so they will get to hear stuff they want to hear. No requests = don't complain about the music.
I love my job, but I do get a little miffed when I don't get tipped, especially if I've spent the better part of two hours scrambling to play all of the requests. I pay for my stream and the software, so it's nice to offset the cost a bit with those tips.
Last night, alone, my four hour set was easily 3/4 requests. I also keep people involved and talking, make sure they're having a great time, and pulled people into the club.
One thing: I never ask for tips, and refuse to use gestures (apart from my "I'm DJ Johnny, please IM me if you want to hear your favorite songs"
. But it is very nice to get tipped, and I'm always gracious about it. I thank each person, and I make damned sure they get their songs played, even if I have to stay overtime.