Hi thebob6,
A mattress protector is usually important to protect the warranty because even a small stain on a mattress usually voids warranty coverage and of course it’s also easier to keep a mattress protector clean and hygienic than the mattress cover. As far as I know though (and you may want to confirm this with them) … Select Foam doesn’t void warranty coverage because of stains on the mattress and because their cover is washable a protector would be less important.
A wool mattress protector like the Dormeir can certainly help with regulating temperature but it will also have a larger effect on the feel and response of the mattress than no protector at all or a thin “membrane” type so in the case of your mattress, sleeping without a protector at all … as long as you are careful about “accidents” … can be an effective option if it doesn’t void the warranty. It will certainly have the least effect on the feel and response of the memory foam and will also have much less affect on the performance of the cover itself. If you use natural fiber or viscose fiber sheets then you would have the best of all worlds (synthetic sheets can also affect sleeping temperature). You can read a little more about the factors that combine together to affect sleeping temperature in post #2 here. It’s usually a combination of several factors that leads to people sleeping warmer on their mattress so sometimes even eliminating one of them can make a significant difference.
Phoenix