parking under tree.. how durable are rubber roofs?

thank you, with that said as far as dirt is concerned, I have heard others never clean the roof because other then not looking good a dirty roof does nothing to hurt it and cleaning can actually wear it down faster then not cleaning it? does this have any merit? my TT is tall enough where you don't see the roof nor how dirty it can be.... it is only a few months old so the roof still looks almost new, but should these roofs be cleaned? or is it true that dirty roofs don't really hurt them?