My experience with spraying down the engine is that sometimes water gets into the spark plug wells. This sometimes keeps the engine from starting, or from running on all cylinders if not all of the plugs gets filled with water.
So just check your plug wells and make sure they're thoroughly dried before you start your engine.
For clarification: he's talking about engines with the spark plugs going down into the top of the cylinder head...what are modern 'hemis,' basically. Though combustion chamber design varies greatly, I think you could call modern heads "hemispherical"...if for no other reason than four valves per cylinder pretty much requires it.
This is opposed to your general 'wedge head' setup, where the spark plugs go in closer to horizontal. e.g. some modern V8's, mostly older V6's and I4's, and of course all of your old 'muscle car' V8's aside the from the aforementioned chrysler 'hemi.'
The latter, you don't have to worry about, really. The ones that go in at the top will have cylindrical wells that pass through the valve cover. There will be kind of an 'umbrella' on the top of the long spark plug boot, which generally seals moisture out...but it's not meant to take even a pressurized garden hose blast. Similar problems are had when the seals between the plug wells are the valve cover are bad, and they get filled with oil. I'd imagine water would be worse.
Nothing should really leak into the engine, though. To get past the crush washer and then threads of the spark plug is a difficult task. I've seen them get oil fouled eventually, though...or maybe that's just because a bunch of oil fell into the cylinder and ran down the plug when i took it out. ;P Your risk is misfires because the spark 'pulse' is using the moisture to find ground before the gap of the spark plug.
This applies to coil on plug as well as distributors.
Again: Simplest response is "no, don't pressure wash your engine."
If it's that dirty, anyway, I'd recommend just pulling the damn thing and cleaning it out of the car while you do a regasket (and possibly a total rebuild, if you fancy). That grime is actually forming a barrier to help keep actual, useful oil from seeping out through your shitty gaskets.
edit: god fucking damn I can write a lot about simple shit. I just thought 'spark plug well' might not be a familiar term to some, especially if they've got an older engine.