I wash mine at least once a month. Purple Power, Simple Green, or any other de-greaser will remove the grass stains and mud from decks and tires. I spray the engine when cool and pressure wash it as well keeping the nozzle far enough away as not to cause damage to anything. These commercial units are built to beat the wet and washing is not going to hurt them. My Toro is 13 years old and looks better than most units I have seen pictures of on here that are a few years old. Granite, I am servicing only about 25 cuts a week doing it part time, but the way some of the folks equipment looks I can't believe they run. I guess if your upgrading every 3-4 years, appearance is no concern. All of my equipment is washed and serviced regularly. Just makes my rig look better from client yard to client yard. Some even ask if I just bought new equipment and are surprised when I tell them how old most of it is. But to each his own, I just prefer it clean. It also helps if I do develop a leak or problem, I can tell it before it gets out of hand where the dirt accumulates.