The grafitti debate pops up every once in awhile. It definitely is a symbol of urban decay, but then again so is rust, junk, busted skids and all that other stuff we try so hard to emulate. If I were modelling modern I would have to have grafitti, but that's just my personal taste. As someone who teaches about such things as "found typography" for a living I look at grafitti as a cool art form.
But the most important thing is that a model railroad is your fantasy world, a place to get away to - and as such, no one can tell you how it should look.
Val