whoa! I don't think it's a good idea to spray WD-40 or lubricant/oilly stuff on your brakes/rotors/pads.
There's some stuff you can buy to put on the BACK of your brake pads that's suppose to shut up the squeal if it's coming from the contact between the calipers and the back of the pads.
If you're going to spray down your brakes and pads, buy a can of aerosol brake cleaner.