I guess it depends what you mean, I have a few special brushes I use just for the underbody and under the hood. I usually wash every week in the warm months and every other in the cold, and I do the underbody everytime. Here's what I do, I wash the wheel wells and mud guards as well as the frame behind wheels and fuel filler hose, then I wash under running boards, whole length of frame rails, each cross member and all skid plates including front. In the back I get the end of the frame, rear diff and shafts as well as the tow package. Then for the real fun, I wax all door jams, under hood and my rims, and yes I know I'm anal.