I prune, the ones that need it, late February/ early March, and feed with Sulphate of potash, water in and the feed again with rose fertilizer or any all purpose food when in bud, and again occasionally through out the growing season.
I now only grow the viticella type clematis as I find they are far less trouble and do not suffer from clematis wilt like group 1 and 2. Mulch generously once you have watered the fertilizer in.
After all the rain we've had the ground will be ready for a good feed.