I'm not going to be in the engine bay. Will be under the cab and in the wheel wells. If there's any left over I might "simple green" the firewall and brush some on there.

If you're asking for your own car, just be careful around fuses and any electrical components. Unless your engine is really cruddy, Simple green and water hose will usually do the job.
I usually spray the engine that's just warm, not hot. Let it sit for 10 mins or so. Then start it up and hose it off with the engine running. Just keep the hose moving.

Here's a link to a good video on engine cleaning. Takes a minute or so to get to the part about engine cleaning.
Cleaning The Motor - Adam's Premium Car Care Products