I do every square inch on mine, they are designed to be wet so as long as your not blasting parts and connection up close you'll be fine.
+1. HF sells a nice gas powered unit with an AR axial pump. The horizontal shaft model, 2500 psi, 6.5 horse Pred motor. Comes with everything but you need to add an additional hose to get some length.
Had mine for at least 10 years now, change the oil in the motor every year, oil in the pump every year too (takes 30 weight non-detergent oil). Keep it inside the heated shop so it never freezes. If one freezes, it ruins the pump, cracks the ceramic plungers.
I do like Wolfman does and I use ZEP degreaser as well. A clean motor and tractor is always easier to service and I dislike filth anyway. Both mine are over 15 years old, both mine are devoid of filth and stay that way.
Use the wide fan tip and apply ZEP beforehand with a sprayer. Let it work a while and wash it off. I've packed all my electrical connections with di-electric grease so no worries about water ingress.