Technically, you don't 'need' a PRV...code requires one if your static or normal pressure exceeds 80psi. If you install a properly sized expansion tank, the pressure should stay pretty constant. BUT, note that if your 'normal' pressure is 74, it COULD rise significantly higher at night when few people are using water in town and the water company is pumping more water to fill the water towers for the next day. In that case, to keep it below the 80psi, a PRV would technically be required. I'd use your static pressure as the starting point. Once you get the expansion tank in, if your pressure gauge doesn't have a tattle tale hand to record peak pressure, pick one of those up. Leave it for say 24-hours, and see if it then gets above 80psi...if it does, add a PRV.