I would say it depends. If you really hose the thing down, you can drive water into places that you never want to have water. If you are careful.....maybe. I would not use the pressure washer like a brush and use the fan spray to clean off the grease and oil in 6" wide passes.
I refuse to use a pressure washer on my motorcycles because you CAN/WILL drive water into the wheel bearings.
I have been thinking about this myself, and I think I would use a foaming engine cleaner and a garden hose, at least until I get the engine out of the truck, then it is pressure washer time on the cross members and frame rails.