ON the heavy 5x5 days, I try to rest 3 minutes between sets, 4 minutes tops. I always try to up the weight, but I think doing more work in less time(ie 25 total reps w/ 315lbs in 15 minutes as opposed to 20) is as important WRT progress and overall conditioning of the CNS and cardiovascular system, and muscle growth as increasing the weight used.
On my light/med 5x5 days, I focus on resting less and less between sets in order to condition myself better for the heavy days. This has helped me tremendously. I rarely rest over 1 minute btwn sets on my light/medium days. IMO, the shorter the workouts the better. I can workout more frequently when my sessions are kept to less than 45 minutes as opposed to 1-1.5hr workouts.
Just my 2 cents.