It depends on if they are ready to be planted now or not. I personally prefer fall planting because it allows my clematis' roots to get established so they can begin growing first thing in the spring. Our springs tend to go from mild to hot and dry too quickly to allow clematis to get as well established as those planted in the fall.

I have said this time and time again on this forum that your own experiences with clematis in your garden and under your own growing conditions best dictate how to handle clematis in your garden. If spring planted ones do better for you per your noted experience, then plant them then.

Well my experience is so limited...how do I determine whether they are ready to be planted now or not, check the roots? If they are not ready, and I know the Bluestone one at least is not, would you overwinter them in the garage or plant them September 1st if those were the choices?

Yep, see if the roots are filling the pots and perhaps even coming out of the bottom holes. If so, they are ready to plant or at least move up in size pot-wise so they don't become root bound. Not sure what your summers are like, but I would wait until late August to early September to plant them. If you want to wait until spring, no need to overwinter them in the garage. Move them up in pot size and sink the pots in the ground. Last winter, in my zone 7a garden, I left clematis I had potted up from Donahues sitting outside until late February and then planted them in the ground.