Quick thought. You're going to want to make sure the ground underneath the deck has a nice slope away from the house. Any dirt you pull for piles is dirt you can use for grading.

Normally, I would say go with a couple feet of gravel and deck blocks, but a 12' x 24' deck is a little large to lift and adjust he blocks after, if need be. If that happens, you'll be very unhappy with the time you "saved" during installation.