You may have your own way of doing it. There is no right or wrong. The government can't control the way we do grunge (at least this statement is still true in my country). Grunge is the art of appreciating the flaws in everything we see in the world. Stain, dirt, scratch and rust are some of the imperfections that make grunge so perfect (isn't it ironic?) If I may summarize what grunge is in my own words, I'll say it's broken, it's rebellious (even emo) and yet it's so awesome! If you call yourself a grunge artist (or pretend to be one), this is one grunge resource on the web you cannot afford to miss. You can find more high quality grunge textures @ grungetextures.com and I have a teaser of what you can expect to see on this awesome site. Below "I'm No Superman" banner is created using some of the textures available on the site. You can download each preview texture by clicking on the image.
Btw, if you're looking for some grunge brushes, you should look over HERE.