Standard butt hinge has three places it can squeak.
First is pin to knuckle.
Second is knuckle-to-knuckle.
Third (least common) is knuckle to some other part of the hinge.
Since the contact points are where squeaks usually come from, that's why I tend to take the hinge apart to get the lubricant in where the noise is (presumably) coming from.
Taking the door off means being able to see if the hinges are rubbing anywhere less obvious, too. And it's a handy time to check any weather stripping, loose parts, etc. All of which are worth doing for a paying customer.
WD-40 gets its name from being a water-displacement fluid, it's a penetrating liquid, but not an explicit lubricant. Pam™ is probably better than the other cooking sprays out there; just do not use Crisco™ as that contains vegetable oil (same as its namesake) which could go rancid on a person.