Late spring flowering clematis cultivars bloom in spring on last year's growth and most of them often bloom again later in the season on new growth. They don't need regular pruning. If, after several years of unbridled growth, the plant is starting to get a little out of hand, you can selectively trim some of the vegetative shoots back in early spring, after the flower buds have formed.
If you want to get fancy, there are a couple of neat techniques you can try. Prune the entire plant back to within two to four feet off the ground and you'll sacrifice the spring show, but you'll be rewarded later in the season with a heavier summer bloom than you would normally get. With the second technique, called relay pruning, you cut half the plant down and leave the other half intact. The unpruned half provides the big early blooms, while the other half shoots up and produces summer flowers. The following winter, reverse the procedure. This technique gives a fuller-looking plant with less bare stem showing.
Summer-flowering cultivars flower throughout the summer on new wood only and may be pruned back hard to a height of four to six inches in midwinter, as the buds start to swell. You'll have a vine covered with blooms from top to bottom. If
you don't prune, or prune back to three or four feet, the vine will resume growing where it left off (or where you cut it off), and the flowers will appear higher up on the new growth. It does no harm to the plant to leave it unpruned.