Why not on the panels?
I'm guessing because CT-18 (or any decent degreaser) will strip any protective wax/sealant coating. Same as washing your car with dishwashing liquid. Truckwash has it's place, but I don't use it on anything that I actually like. Proper carwash is cheap enough from SuperCheap.
I just use the pressure cleaner on the engine bay. If it's real dirty, I give it an all-over sudsing first, and let that soak for a couple of minutes. With the car running, the heat of the motor does most of the work for you. Just don't close the bonnet - you want any steam to be able to escape easily, vs work it's way into connectors, etc. WD40 on a rag will polish up everything that needs to be polished after you're done. I've never had a problem with it attracting too much dust, but I let it dry fairly well and use it very sparingly.
Just keep in mind that with the new generation of diesels there are electronic gizmos that would rather look a git dirty than have a long, sweet, chemical bath.
People say that about washing petrol engines too, but I've never had a problem. So long as you don't blast a connector block, fuse terminal or distributor with the pressure washer, my experience suggests to me you'll be pretty right. Of course, I may be wrong, in which case look for a suprisingly cheap Pathfinder coming up for sale in the near future with a spotless engine bay (can't hear running though).
Cheers!
Matto