I just want to add a little to the good advice pgmr gave. Temperature/pressure relief valves aren’t supposed to open below 150 psi. But anything above 80 psi is extremely destructive to water heaters, piping and appliances.
So if you have pressure, even spikes, above that, you do need an expansion tank. If you get one, it should be installed on the cold side and below the tank so hot water can’t rise into it by convection. The rubber diaphragm will fail sooner if it’s doused in hot water.
Also, tanks come precharged to 40 psi, but they have to be charged to line pressure to work properly. Thus, if line pressure is 70, they have to pumped up by 30 psi to match that.