I aim to get the pressures right on before setting out for the day. +/- 2 psi back (which, for my RV, need to be at the maximum 80 psi I think...I do need to get it weighed properly soon to verify), +5 -0 on the fronts which should be within 2 psi of each other.
I don't think it's been mentioned yet, but it is rather important that each of the two tires on the same side of a dually (i.e. both the left tires or both the right tires) be at as close to the same pressure as possible to avoid undo wear. This is more important than having the pressures match on either side of the differential. The reason is pretty simple if you think about it: if the tires in the pair differ in pressure, one of the two is of necessity slightly larger around than the other, and since they must rotate at the same speed, one or the other will have to slip ever so slightly as you're driving straight down the road. I try to keep the tires in each pair within 1 psi of each other.