I wouldn't recommend using a power washer. If you use high pressure water, its only going to increase the chance of water penetrating your sensors and/or harnesses. I wash engines on a regular basis, and I find just engine shampoo and regular hose pressure is enough. Don't forget to cover up and protect certain areas of the engine bay if possible, such as the alternator(really hard to do on our duratecs, I just avoid spraying in that area) and coil packs.
...after reading your title to this post, the last thing I thought this topic was about was washing engines...
My mind is always in the gutter, so you'll have to excuse me!
