It depends on the condition of the roof, how it was made, and how much you weigh. As equipped, I weight 157 pounds. I walk all over the roof of my fiver and my TT. Both have 1/2" decking. On the fiver, I can feel it giving under my feet, and if I weighed 250, I would stay off of the roof. My TT is solid as a rock and I would not hesitate to walk on it if I weighed 300 pounds. If the roof ever leaked and the decking rotted a bit, I wouldn't want to set foot on one of those spots no matter how much I weigh.
BTW: 303 Aerospace protectant seems to slow down the accumulation of dirt on the roof and reduce streaking. It is safe for rubber roofs.
If you don't want black streaks keep the gutters clean and have drain spouts....AGAIN the membrane is designed to have that "sluff" left alone NOT removed. HOWEVER...if you have a TPO roof washing is fine as it does not sluff off since it is vinyl