If it was me..I would let them grow up and let them bloom, so you can enjoy the first flowers and after the blooms are all done ....then hard prune..remember to fertilize too after pruning...this would be done so your clematis can get a stronger root system..then next year "Let 'em rip" and treat as the pruning type that they are....Jeanne

If the plants are starting to bud, I would do what Jeanne suggests. If the plants are just starting to emerge and have not yet produced any buds, I would pinch out the growing tips now to encourage branching and more places for the plants to produce buds. This would even make more sense if your clematis are type IIIs since they bloom on new growth. Pinching now would not affect their blooming at all other than perhaps delaying it until later in the season. I would still do it to your type IIs since it will encourage a stronger root system and the plants would be better off in the long run.

I agree, I've pinched everything that didn't have flower buds already, no matter how long they have been planted.