Careful there – slightly warm is O.K., but do not start with a HOT engine. Not only do you subject components to unusual thermal expansion stresses, but starting with a hot engine will (potentially) make things worse. A hot engine will produce water vapor or steam; this is more likely to penetrate into areas that you do not want moisture.
Usually, there is nothing to worry about. Common sense is really all it takes, although some cars seem to be more susceptible to moisture induced problems. Personally, I cover the brake fluid reservoir cap with plastic wrap & a rubber band – there is a vent hole in the cap, and brake fluid being hygroscopic & all…. Common sense also means *not* blasting a high pressure stream of water at sealed electrical boxes, such as ignition units and relays, amongst others.
On gasoline engines, once you are done, remove the distributor cap and dry out the inside surfaces & the wire terminals as well as the rotor. Misfiring may still occur due to moisture at the spark plug end of the wires, or along the wires themselves if they are getting old (read: bad). There are several spray products that are used for electrical system water displacement, although I have never used them. They may be handy if water becomes a problem with other sensitive devices however.
And don't forget that while your washing away the crud, you are also washing away key lubrication at throttle linkages. When I wash the engine, I take them off to clean them and then lube everything when I'm done.