If you use WD-40, all that WD is going to have to go somewhere, like on your driveway and onto your lawn. Don't really like WD - there's almost always a better product for any job that it would do (although it's a good cutting fluid for aluminum).
I've done a lot of engine compartments with simple green. It might take a little more scrubbing than something harsher, but I don't have to worry about as much. At $10/gallon, it's reasonable. The only hazardous waste is whatever was on the car.
Castrol purple degreaser is good, but it can eat paint and aluminum, so I'd only use that where appropriate, like steel parts off the car.
Gunk engine bright is good it you're looking at really grimy stuff, but it might be overkill for just a normal cleanup.
Steve