The fuse box should be able to accept a little moisture as long as you don't continually blast the box with a high pressure pressure washer.
I didn't warm up the engine. I low pressure sprayed with some commercial grade degreaser over engine. Waited a few minutes and then high pressure sprayed and got good results. Be wary of stickers flying off the body if you use to high a pressure washer
(I blasted one of my fuse box diagrams off). Also, drive belts may become dry and begin to make noise.
I started the enigine afterward and drove down the block to dry off.
Remember to pay extra attention to electronics and air intake areas, such as covering, and or avoiding prolonged direct conact of high pressure spray.