The main reason just about all pressurized aircraft hold a bit of pressure is to keep the doors and windows from rattling or whistling. I forget when the 747 classic did it (someone above mentioned kt) Most aircraft get their pressure bump when throttles are advanced past T/O warning logic switches or positions. Not sure of this but they probably dump when air/ ground switching is made.
Future aircraft will pressurize to greater than 8.5 DP to lessen the altitude effect on Jet Lag, they want to bring the cabin down to less than 5,ft. Modern corporate jets are capable of this to achieve the 40-50k ft cruise altitude. Saves fuel and gets you to your destination faster. Not to mention clear air space high above RVSM.