i usually go with the flow of traffic, for several reasons:
1) roads have a "natural" speed, i.e., a speed at which driver's feel comfortable... and that is usually the speed that you will see people driving, regardless of the posted speed limit. e.g., a twisty back road may have a speed limit of 55, but people may drive on average 45. on the other hand, a freeway may be posted 55, but has a "natural" speed of 75.
when roads are constructed, there are supposed to be extensive studies conducted to measure the average speed of people on different stretches of the road over a period of time. that's the "natural" speed, and is supposed to be used as the basis for determining the maximum speed of the road, before also taking into account things like zoning laws... many times these studies are simply not done. just today, i was heading on a stretch of I-90 leading out of boston, a 6-lane highway with dedicated median and no street intersections. the posted speed limit was 35, but i swear you could comfortably go 80. in this case, i kept it down to about 45 to avoid a ticket (it was pretty empty, too, so i didn't want to be a target).
2) contrary to what you might think, speed limits are not always scientifically determined based on the usual factors such as visibility, curves, hilliness, blind spots, school zones, etc... sometimes they are what they are for now logical reason. also sometimes they are what they are for political reasons.
3) if the flow is going one speed, and i am going another, i am a hazard to others just as they are to me.