I wouldn't cut them back until late winter or early next spring. That will stimulate new shoots to emerge giving you a fuller plant. Cutting it back now would hurt your clematis. It is a young plant and it is trying to establish a good root system this year. Taking half of it's food source away by cutting it back could cause root die back. Just leave it alone and keep it well watered. Clematis are long lived plants and take 3-5 years to get well established.

I disagree. It won't hurt to cut back your clematis now - cutting back top growth actually stimulates root development, even during the prime growing season. You can safely take the vines back by a third. Plus with some type 2's, a lighter pruning after the initial bloom flush will encourage a later rebloom.

I'd wait until late winter/early spring for doing the recommended hard prune - you could do it now but would need to repeat the procedure again at that time anyway.

flowerfan, we cut back the potted clems at my nursery multiple times a season, both to tidy the vines for maintenance issues, encourage additional flowering and to stimulate root development. Cutback vines actually produce a denser root system than those that do not receive this treatment.

I agree with Gardengal. Don't hard prune them all the way to the ground but if you take off about a third you may get more blooms and it is beneficial.

Thanks so much for your replies!