This isn't specific to cuben (which I've never used), but I like to roll. I've heard people say to stuff (ie. "Why do you think it's called a STUFF sack?") but doing so puts a lot more creases/folds in the material. When you pull out a stuffed tarp the whole thing is covered in creases, whereas a rolled tarp only has a few.
Does this really matter? I dunno...creasing seems like it can't be a good thing. It may fatigue/wear the material over time.
Besides this, it's easier to pack it smaller by rolling it.