Regular PVC does tend to start to soften above 90 degrees. If a spa is being heated, the water will regularly be above 100 degrees. Since there are so many different Mini-Max models, some with bulkhead fittings, some with cast metal headers, a good recommendation is to use a short section of CPVC on the inlet and outlet ports. I always do this on my heater installations and have never had an issue with the connections directly at the heater unless there is a malfunction in the heater itself. A couple of short pieces of CPVC (6"-12") does not cost much and is, in my opinion, worth the extra cost and effort.

Unless you have a "fireman's" switch on your timer (for heating a pool) or an auto control with a cool-down cycle, most people will not cool their heaters properly after using them, especially with spa use. Even the retained heat in a newer heater and plumbing will eventually warp the systems plumbing if not cooled. Heater manufacturers say that proper cool-down can add 50% to the life of a pool/spa heater.

Just Thursday had to re-plumb a system that was only 4 years old but had very regular spa use. It is a manually-controlled system and the customer said he was never told about cooling the system before shutting the pump down

Just my experience.