Should I prune back Clematis to lowest viable bud or highest?
Assuming that your clematis is the kind which blooms on new growth each year and therefore can be pruned back hard in the spring, you could prune it to the lowest bud if you wish. Going down to about two buds from the ground will produce a nice coverage of leaves and bloom display. Be sure to trim out any damaged ends and winter kill.
However, there are three kinds of clematis and the others would not want to be pruned back that far in the spring. Please check the more detailed link given below for the whole story. If you don’t know the name of the variety you have, just think back to your past experience with it to see which pattern it fits best.