We have pine siding, some as old as 20 years, some as "new" as 15 years. It has all been stained with a transparent stain, but needs to be redone. There is some mildew near the bottom and much has darkened over the years. How do I best prepare it for re-staining? I have access to a pressure washer, but have read that it can raise the grain and necessitates sanding before staining. Your opinions?

Some pressure washers can actually cut the wood deeper than just raising the grain. I would spray the house with a mildew/mold killing house wash and the pressure wash with 2200 psi or less washer.