We had our house painted 2yrs ago & it was pressure washed & then spray painted with Sherwin Williams paint. I think the worst thing about it was all the paint everywhere that had come off the house, keep yr vehicles well away if you go that route.

 

I got my house painted last year and they pressure washed it and sealed all the cracks, etc. the day before. Then they came back and painted the next day. They sprayed the gutters, soffit, and facia boards. Then they rolled the rest of the house.
That was on a concrete block house with stucco, so I'm not too sure what to do with siding. A house around the corner from me has siding, and when it was time to repaint it they just tore it all off and replaced it. Looks real nice now.

 

Just hose it off, you don't need to blow every speck of dirt off of there. I have a friend that is a retired pro-painter and that is his recommendation. Any dirt that remains will wind up suspended in the new paint with no problems.

But you do need to be sure that any existing loose paint is removed, obviously.