A lot depends on how much cleaning they need. You are a chain smoker or have cats treating the grilles as love objects, you can use something like Simple Green or Oxyclean. You are a pristine housekeeper in a climate- controlled space, some mild soap and water will do it. Just put them in the bathtub, plug the drain, and use the shower head. I've found with virtually every form of upholstery cleaning, the RINSING is as important as the cleaning. Many soaps act as dust magnets, and the cloth gets dirty much faster if it's not removed. Most of my grille frames are the fiberglass or plastic composite that would stand up to a lot of detergent. There are some great threads on here in which people talk about their choice solutions, and I bet you can find a technique that matches your grunge level (RIP, Kurt Cobain).