Do Clematis plants ever need to be trimmed orcut back, if so when?

Wait till the buds break open so you know what's dead and what isn't. Then you can prune it to your needs. If it's sparce at the bottom (clematis sometimes tend to be) prune the top some to force out more lateral buds.

Do Clematis plants ever need to be trimmed orcut back, if so when?
Generally speaking clematis likes to form clumps and drop its botton growth. The higher the vines, the more likely you are that the flowers will form and clump higher up the vine each year. To keep a full plant with blooms from the bottom up, cut the plant back to the desired length when the plant has gone dormant in the Fall/early winter. New shoots will come up from ground level if you have cut them back to within a few inches of the ground.
Reply:In reality there are 4 classes of Clematis and each has different pruning needs. That said, I totally agree with the answer from College. I use that method every year and it works well. Here I thought I was the only one who knew! lol