I think a good rule if you're just starting out or haven't been in the game long enough to know an in-depth history of writers here is this - DON'T GO OVER ANYONE..... especially if it's faded. Does it seem odd that the piece is left alone in its current state of fadedness and nobody else painted there but everywhere around it? There's probably a reason for that. If you've never heard of them before, it doesn't always mean that they aren't worth leaving to see another day. You can't just give yourself the benefit of the doubt and believe that your toy ass knows everyone that matters or who really came before you. Chances are there are a nice handful of pieces out there floating around from oldschool cats that you never heard of and we as a community would much rather see those continue hanging around than be covered up by your toy garbage..... I mean you're really not impressing anyone with your badass graff skillz(which was likely your intention in the first place). Instead you're just showing how much of a dumbass you are and how little you know about the history of those that perfected this craft long before you even though it was cool.
Also... If a piece is dissed out or fucked up and you haven't been painting for long or have a good rep for yourself it might not be a good idea to just go ahead and take that spot because of the fact that it was dissed out. In that particular scenario it makes it look as though you were the one that dissed it out or fucked it up just so you could come back later and say hey it was dissed out and take the spot. Just let it ride... especially if you can still make out what it says. Chances are that person will eventually come back and fix their spot anyway and if not then oh well. There are a million places to paint in this city.
Ignorance is a sad sad thing, kids. Don't let yourself fall victim to it.