I've been taking pictures at con for years. Sometimes I ask, sometimes I don't, but rather rely on the person to indicate if it's OK. If you raise your lens in someone's direction, a quickly turned head or a raised hand will tell you well enough to stay your trigger finger. (This is useful practice in places where it's not practical to ask, such as in the drum circle or club dance.)
In all this time, I've never once had a problem with a fan. Most of them are more than happy that you're paying attention to their outfit, dance moves or whatever and may even ask for copies.
But certainly be wary about crossing the line when someone really doesn't want it. Above all, listen to your subject. You don't want a crummy picture of someone's scowl anyway.