It is not a requirement but most clematis growers recommend it. You will have a tidier, more manageable plant that often will produce more flowers than one left unpruned. It helps a lot to know what kind of clematis you have to determine when and how much to prune. If you don't know, paying close attention to bloom time can help. Very early blooming clematis (before May) generally require minimal pruning unless you want to control size. They bloom on old wood (growth produced the previous season or before), so if pruning is desired, they should be pruned as soon after the bloom cycle as possible so as not to affect flowering the following spring. Mid season clematis - those that bloom in May and June (and sometimes into July) tend to bloom on old wood and sometimes as well later on newer growth. These are typically pruned lightly - about a third - in early spring, around the same time you prune roses in your area. Often, many growers will prune again lightly after the first bloom to encourage a later rebloom on those that flower on new growth as well. Late season clematis (July - September) are best approached with a harder pruning. These bloom on new growth and if left unpruned, can produce long, leggy vines with flowering clustered only on the top of the vine.

It sounds more complicated than it is :-) The vast majority of vines fit into the second category - the large flowered hybrids - so that should simplify things. And pruning instructions are only guidelines, not requirements.

If the vines are newly planted, any clematis regardless of type benefits from hard pruning down to the third set of buds at the end of its first season (late winter/early spring). This encourages the development of a strong root system and the generation of multiple stems from the root crown and often, increased resistance to wilt. A good many growers will continue this practice in the second and sometimes the third season as well.

It is hard to hurt an established clematis by pruning regardless of type, so if your vines are large, messy or unwieldy, pruning at any time can provide a necessary rejuvenation. Pruning at the wrong time or harder than typical for that type of vine will only impact flowering time and usually only for a single season.