What should be the maximum PSI on the cold water line while the water heater is heating?
The maximum PSI for a home water line is 80PSI. Low water pressure is below 40 PSI. You typically want 50-60PSI for your home. If the water pressure is rising while the water heater is running, you need to install an expansion tank (as Wallace mentions, +1). If the water pressure is always high, then you need to install a pressure reducing valve in addition to the expansion tank. It's possible for both of these to fail over time, so if you see symptoms indicating that you don't have one of these, then consider replacing it.