Disagree with everyone, I was at polished bliss the othernweek and got some good advise from them and even though ther website says use an open ended hose the guy told me to use a pressure washer as it doesn't leave as much water lying around.
Obviously don't go crazy blasting everywhere but a good general clean where needed and then as soon as your finished fire up the engine to aid the drying process.
As long as you sensible with where your washing then all will be good, been using power washers on my engines for over 10 years and never had any problems.