Hi tcornflake,
Select foam is one of the few manufacturers where a smaller stain on the mattress doesn’t invalidate their warranty but if it becomes abnormally soiled or unsanitary the warranty would still be voided. I think a mattress protector that can be easily removed and washed is always a good idea to protect the mattress from the moisture, fluids, and skin flakes that we release each night, protect it from accidents, and to keep the sleeping surface cleaner and more hygienic.
Post #89 here has more information about the different types of protectors and the tradeoffs involved with each.
Phoenix