From my limited experience (one house built) I would strongly advise the use of sills. I didn't and have regretted the decision more than once. For a four post frame it might not be useful, but the moment you need to align multiple posts (ie. 3 or more in a row) the sill saves a lot of effort. You could always still pour a slab, and make a ramp over each of the sills for the entrance... although this would preclude ever pushing a vehicle into or out of the garage!

If you're planning on raising as bents, bear in mind that your centre posts create a hinge point for the girts. Either you have to size the joints so that they can take the lateral forces and lift the girts, or you have to lift all three posts simultaneously ensuring that the force is distributed evenly... again... this is the voice of (recent) experience! :-)