If the condition of the roof is poor, walking on it is going to break up the shingles and leave you with a roof that looks like it just got machined-gunned. It may leak that way too after walking on it. But, if it's in a state of condition where the shingles are somewhat flexible and in-tact, walking on it is fine. Of course, I wouldn't walk on it like you walk on your driveway every morning to get to your car. In other words, if you're walking on your roof to get to a widow-watch on a regular basis, don't because you'll definitely age the shingles! The newer the roof, the less likely you'll do any damage walking on it. BUT, I'd still tread lightly. Oh, if it's steep, walking on a new roof could damage it because, well, gravity is going to want to pull you down along with the shingles. Friction has a funny way, you know?