As with any time you use a pressure washer, be careful about how much pressure you apply. Too much pressure and you'll likely wreck it. You don't want to blow off the backing or cause the pile to separate from it.
I'll repeat my recommendation to use an actual carpet cleaner. I've had great luck using one like this:
http://www.amazon.com/Hoover-SteamVac-C ... B00009R66FBoth for the boat carpets and several at home. It really does a great job. Both in cleaning the carpet and in vacuuming back up nearly all of the water used. As opposed to a pressure washer where you're going to have to complete saturate the whole rug to push the dirt from the center all the way out to the sides. And then have it sit out for several hours to dry. With the cleaner they're only slightly damp right after cleaning and dry within a half-hour.
So it can't hurt to try to the pressure washer trick to see if it reinvigorates the pile. But for regular cleanings I'd go with something with a lot less destructive power.