Our last water heater (gas fired) lasted only six years before it started gushing from a crack at the inlet. I tested the static water pressure in our system and it fluctuates between 68-72psi depending on time of day. It spikes slightly to 80-84 from valves closing (toilet, washing machine, etc) but does not increase from thermal expansion (there is no expansion tank, but there is no check valve in our supply, either, and I tested this after taking a shower and not using any water for the next hour). The pressure quickly returns to 68-72 after the short spikes from using faucets, etc. Other than the water heater we replaced, there have been no signs of high pressure. No water hammer, and valves and pipes have not had problems, and some of the valves have not been serviced for many decades.

I've read that 80psi is the highest recommended pressure without a PRV. Is that based on purely static pressure? Or does this include the normal spiking from valves closing? Does my pressure warrant a PRV to prevent future issues? Or am I looking for a problem that doesn't exist? The six year failure of the water heater triggered the question.

That pressure had absolutely NOTHING to do with the water heater failing, and you do NOT need a prv to regulate the pressure.