Generally speaking, graffiti writers (at least here) go by these rules. Although they aren't limited to them:
No religious buildings
No houses or cars unless it's abandoned
Commercial buildings and commercial property are OK, public property is definitely OK and usually the main target.
I grew up around all sorts of graffiti, so it feels like home when I'm in an area with a lot of it.
Gang graffiti is not art, it's purely to mark territory. It's easy to distinguish gang graffiti from what a graffiti writer has painted. Much easier if you already know what gangs are around the area you're looking at.
Notice how I didn't say graffiti artist. Some graffiti is art in the traditional sense, yes, but the majority of graffiti writers are bombers. They go out and hit an area as hard as possible to get their name out. The bombers I know have enough talent to go out and do some serious painting, but they usually don't bother. I do consider bombing 'art', but in a weird, urban calligraphic sense.
As far as what people think, most of the graffiti writers could care less. Getting all huffy-and-puffy about it and re-painting the wall with a big, ugly mismatched splotch only makes it look worse. It'll probably be hit again soon afterwards if the writer cares enough.
And for the record, BANKSY IS NOT THAT GREAT. And he's NOT A GRAFFITI ARTIST.
He's a street artist. Graffiti writers don't use stencils.