I try to not exceed the speed limit by more than 5 mph. If you do the math, it rarely makes sense to speed. In the city, you are at the mercy of stoplights and traffic anyway, so speeding isn't all that likely to make much of a difference on when you get to your destination. Obviously, if you get stopped by a cop, you will get there later than if you had not sped.
On the highway, you can make up some time on a very long trip, but let's say your destination is about 2 hours away and the speed limit is 65. If you drive 70, you have no significant worries about getting stopped. If you drive 80, how much time do you save? Somewhere around 20 minutes, I think (too lazy to do the math). Once again, if you get stopped, you will lose more than that 20 minutes, plus you will have to pay an expensive speeding ticket.