So, you want to do the Bed - First - and get all the water, overspray, dirty soap, etc., rinsed out of the bed and out of the way..
While you are up in there, you can clean between the bed and the back of the cab, at least 1/3 or more of the truck roof, and might as well take a claybar up there or whatever you use and do that to the top and back of the cab while you are there... Dont forget to hit that back glass if its really dirty as most are since the air blows back toward the back of the truck bed when in motion..
Consolidate as many jobs as you can when you are up there if possible, and it will always be easier later..
I always wash the bed from the front closest to the truck cab back to the rear and out, using big brushes, brooms, pressure washer, and plenty of soap..
No one expects a Detailer to polish out a painted, dirty, nasty, scratched from loads of rocks, etc., bed, and are all happy if its just cleaned out really well and looks presentable, depending on the media on the bed and bed sides, back of tailgate, etc...
Dan F