Do you size a DHW expansion tank to incoming pressure or pressure after PRV?
PRV (or pressure regulator) failure should be a rare event. 

One problem with pre-pressurizing the tank based on the incoming pressure is that it will accept essentially no water until you reach that pressure, so that seems like a poor idea. Whether the incoming pressure will be so far above the functional pressure that it will pop the expansion tank bladder will depend on the pressures in question and the tank. I suppose the approach to take would be to figure out how far to pre-pressuirize it to prevent popping in the event of regulator failure, but that is not actually covered in the usual installation instructions.

If there is a very large difference in pressures, you might want a relief valve on the low-pressure side. This will of course depend on your local system pressure and how far down you are reducing it.

Responding to 90/65 psi comment - this is probably well within the capacity of normal expansion tanks. A typical well tank is expected to cycle over a 20 psi range, but the actual volume in the bladder changes (20/40 PSI holds more water than 40-60 or 50-70 with correct pre-charge pressure.) Given typical upper working pressure above 100PSI, you should be fine pre-pressurizing to 63 PSI (or 2 PSI below whatever you actually set to) and not pop the bladder (it can work at 20/40, so at 63 it can hold enough water to go to 126, even though that's an atypical use-case; so 95 should be just fine.)

A 100PSI relief valve would still be a smart move (i.e. don't make the T&amp;P on the hot water tank the only relief in the system) to protect against a higher than expected input pressure and regulator failure, but might be moderately paranoid (I get that way in systems design, at least where it does not cost too much.)