It was below zero last nite, so hard to talk about power washing this time of year 'here'.... ;)
I like to blow off the combine with air, don't like to force water into it and then park it away damp. It typically is below freezing more than not by the time we get done with corn.
I have power washed it after oats harvest, in early fall. Or closer to early to mid soybean harvest. Actually I wash it the evening before I'm done, let it dry over nite, use it to finish up the next day so all the gunk gets worked out/ dried out, then blow it off to pack it away. In the heat of early fall that works well 'here'. The air gets alot of the fresh dust off.
Like the others, I don't like to make it soaking wet and pack it away that way.