Of course there're many factors involved, but traditionally, I follow the rule that exists in many places I've been (maybe nationally?), that hits you with a suspended license if you're convicted of going more than 15 over the limit and faster than 55mph. So in clear conditions, I go exactly 14 over, but will go faster should the need to pass arise. My commute is a pretty wide open road with an impassable train yard on one side (so no side streets on that side).
Residential areas, though, I won't go over the limit, period. On streets with side parking where I kid or animal could jump out, I'll do as little as 20mph even though the city limit is 30.