I always drive five miles over the limit, except in residental 30 mph areas. I always obey the posted speeds in neighborhoods. Most times, on major streets, the flow of traffic is at least five miles over.
In Houston, we have some very big roads, and depending on the situation, I might go ten miles over. It all depends on the other traffic. If it's crowded, I won't try to speed through, but if everyone else is going that fast, I don't mind keeping up.
On freeways, it all depends on traffic. Posted speeds are 55-65. Most times, I go 70, unless it's crowded, in which case the posted speed works fine. In very light traffic, the highways usually move at anywhere from 70-80, so I'll at least keep the pace. Late at night, with no one around, I'll go 90, but if anyone else does enter the freeway, I respect their safety.