Doing the underneath depends on how serious you really want to clean it. If you want to you can get under there with a rag and cleaner. I'm not quiet that enhusiastic most of the time.
To do underneath I usually go to a DIY car wash and spray the underneath with high pressure soap. Then rinse. Go home. To dress it up and make it look real black, I spray the underneath with
Griots Undercarriage spray. It's basically like a thick tire spray. It makes the truck look Showroom clean.
One part that's pretty easy to wipe down is the frame rails, which are fairly visible if you don't have steps on the side of your truck.
