In Australia, it is reasonably safe to go about 10km/h faster than the speed limit. I have been doing this for about 3 years now and have not gotten a single speeding ticket.
mblase is probably right in saying the speedometer can be slightly off. Between the error in the measurement of the axle speed and the variation in the tyre radius (think different tyres and tyre wear). Given that and the error inherent in the laser or radar gun that the policeman (or woman) is using to measure your speed, I figure up to 10% error is reasonably acceptable. I tend to round up, so I end up going 70 in 60 zones and 80 in 70 zones ... but up to 125 in 110 zones ... :-)
My other observation has been that a significant number of cars indeed go slightly faster than the speed limit on the major roads. I have found that, on 3 lane highways, most of the cars in the middle lane are often going at 10 above the speed limit and it is probably a reasonable assumption that the police are not going to give speeding tickets to the majority of the cars. Leave the slowpokes in the slow lane behind and follow the speed of the cars in the middle lane if they're going not much more than 10% above the speed limit, I say. It will be the speed freaks in the overtaking lane that get caught.
Having said all that, this tactic does not work against speed cameras, which are often unforgiving and will catch you (at least here in Sydney) for going more than 5 km/h above the posted limit in their area. Thankfully, I have always known the speed cameras to be obvious and to have signposts warning of their presence anyway.