First, a quotation from Paul Graham:
Graffiti happens at the intersection of ambition and incompetence: people want to make their mark on the world, but have no other way to do it than literally making a mark on the world. I'm talking here about everyday tagging. Some graffiti is quite impressive (anything becomes art if you do it well enough) but the median tag is just visual spam.
For the most part, we agree. Any idiot can take a can of spray paint and put his initials all over Charleston. We're not looking for these types of tags. However, if the tag required any amount of artistic skill or ingenuity than it is something we want to capture. Some tags can be simple yet still clever (like a keypad stenciled on a wall); these still require some creativity and are worth preserving. Use your best judgement; the only pictures we will ever remove are the ones that are not of tags.