I work in detail at my local Toyota shop (family is great to have in the biz) for the summer and on our major details, we DO powerwash the engine bay. Be sure to not get close to any of the parts.
If you get a good 3-4 feet away from the engine, battery, ect, and use the part of the spray jet at the end where its not so much pressureized.
It is safe to spray all of the components under the hood. What happens when it rains anyway? Engine degreaser is a good thing to have in the garage. You can also uses it on the front bumper and windshield to get all the bugs off from highway/country road driving. IF you choose to use engine degreaser, make sure that you keep the car wet. You dont want that stuff drying up.
Good luck on you engine cleaning. You could also use a regular hose without having to worry about pressure on parts.