Don't pressure wash siding!
Just don't.
There are many reasons, which range from the professionalism of the people doing the pressure washing to the damage pressure washing does to siding, paint and trim work.
Case in point - I saw a house in my neighborhood being pressure washed.
The temperatures outdoors weren't exactly suited to such activity, but this house, on the market, must have had someone think this would help sell the house.
Here the pressure washers are trying to remove a stain under a gutter at the upper edge of the brick.
After they were done the stain was not removed.
But even still, when finished there were problems everywhere.
Having a quick look around here is what I came up with. It's not good, and easily understood.
Unbelievably they decided to spray the paint on the foundation wall. The paint was removed, of course, with peeled paint chips all over the yard. Do they intend to come back and repaint the wall? Winter is NEVER a good time to paint. I've had posts on this before - read one here, part 1 of 2 - and believe me it is not good to paint in winter. When temperatures are below 50-60F (read the label on the can!) and the humidity above 50% the paint will not adhere, and if it freezes before it dries completely it will bubble and peel quickly. The future buyers of this house would be inheriting a mess.
Also, look at the wood trim above the stain on the brick work. Holes were gouged into the wood, damaging it. Why pressure wash something in order to replace it later? Since that doesn't make sense it is apparent that they intend to "repair" and then repaint this trim too. Utter silliness.
Finally, and this is particularly disconcerting, a day later I looked and water was STILL coming out from inside and behind the siding.
What does that say?
That pressure washing this siding, which is a temporary fix at best for staining, the tip had an inappropriate jet angle (meaning less than 18 degrees), or was held too close, and the pressure was WAY too high.
Not only can that damage the siding, but water, and whatever cleaning agent, if any, was forced into and behind the siding. So whatever the material is behind it (and when this house was built they did not use plastic house wrap) was infested with water and left very wet. Which means it froze during the night. Which means it did not dry well. Which means other problems can result. For example, if the insulation was rendered wet it has become compromised.
My recommendation: pressure washing siding sets the house up for problems! And some poor unsuspecting buyer will be the beneficiary of those problems. And in speaking with pressure washing "experts" about this they understand this and agree with me! But it's their business, so they persist in doing what they do. And every time I have a post about pressure washing I get ripped by these "experts" telling me I don't know what I'm talking about. Well, look above and decide for yourself.
Don't pressure wash siding!
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
