Unless something is leaking, the pressure due to expansion will exceed the incoming 90-psi, or try to, so your pressure during/after heating water will get to 90-psi, and stay there until you use some water. If your PRV has a bypass, it should stabilize around that 90-psi, but that still exceeds the residential maximum recommended, i.e. the reason you put it in in the first place! Where I live, after a cold spell in the winter, my incoming water can be literally just above freezing. That means, it's got to rise nearly 100-degrees, and I've got a fairly big tank, so without an ET...there would be leaks every time the WH ran.
Do yourself a favor and put in an expansion tank...in combination with the PRV, it will keep the pressure nearly constant over time...yes, it will still go up some, but it won't spike like it would without one...and, if you go with one a little larger than the minimum, and the pressure rise will also be smaller.
In a really tight plumbing system, after a PRV has been installed, your T&P valve should open. Often, something leaks and a common one is a toilet fill valve...you might hear it, but since any excess water will just go down the drain as the bowl continues to fill, you may not notice. A dripping faucet may be big enough to prevent the pressure from spiking.