Help me figure out my wood shed foundation!
Digging a little deeper and setting the piers on gravel seems to help them heave less, but if you go below the frost line they should be pretty solid.
Digging to that depth by hand is slow work, the first foot is cake, the second foot a little slower, the third foot quite slow, the fourth foot is like digging with a table spoon.  

You could support the five 4x4 rails with a post / pier at either end, and one in the middle;  that's 15 piers, a lot of work.  It will be practically like digging three 10' footers.  

You could use six piers, four in the corners, plus one in the middle of the 12' sides, and build three 10' beams on 12' rim joists.  That's a manageable amount of digging and building piers but you have to frame the joists / beams and the shed's going to sit higher, you'll need to build a ramp.  

I think because the shed was made to set on a level surface, you're going to be better off pouring a pad.