Rooftops can be tempting, and the challenge of getting up there can even be fun (see: buildering), but largely, it's just a bad idea. I won't say that I haven't been on my fair share of rooftops, but it's often dangerous, there's nothing really good up there, you risk attention from the authorities, and getting down can be dangerous.
Really... it's not worth it. If you have to, be safe, pick a good time to do it, plan how you can get down safely before you get up there, and don't do it often.