If you're using a pressure washer make sure you don't use too much pressure... just enough to get the water up there. You don't want the pressure of the water to break through any seals under there.Ehh give it a try. I can usually do it without getting wet. The pressure washer wand is longer enough so that any splashed water won't hit you.
At least for me. And I find its more thorough than those underbody washes.