[John Willis]

The greater the pitch of the roof, the edges of composition shingles can be torn up by heavy foot traffic on hot days.
Runoff from high-pressure washing can also temporarily stain shingles.
If the painters can strategically lay plastic during the power wash and then drop cloths during the spraying (or cut-and-roll), it would help keep the paint drips and foot wear to a minimum.
The safety of the painters while they're on the roof is important, too.

Also, if you have new metal flashing, roof jacks, etc., it's good to make sure those surfaces are clean and wiped down with vinegar (it lightly etches the surface) before priming. The paint has a better chance of not peeling off within the first year.