Donna, if the clematis are well established then you probably won't do much harm with cutting them back now other than you will be removing wood that they would bloom on next spring. However, next spring the plants will put out more new growth which will harden off and give you flowers later in the season.

Generally speaking, you don't want to cut back clematis now since it will stimulate new growth which taps into the reserves that they are storing for next year's growth. The further north you go, the sooner the first killing frosts will arrive and knock back your clematis. So pruning now will stimulate growth which will get killed back. If the plants are well established then this shouldn't hurt them much at all. If they are newly planted young plants I would wait until nature naturally puts them into dormancy to cut them back.

Since they bloom on new wood & are type 3 pruning cut them back. I have done it before to established plants without any problems. You might get some new growth but with as fast as our fall frosts are upon us I don't think you will see any damage. My plants are all putting on new growth as we speak because we are getting rain now after a long drought. Just don't fertilize them now.