While it is not exactly a "rule", it is generally considered to be a very sound practice. The hard pruning of any newly planted clematis encourages the development of a good root system. And with pruning group 2 clems - those most prone to clematis wilt - a well-developed root system can go along way to discouraging this issue.

I would recommend cutting your new clems back hard in late winter. I might even consider stretching that out to the following season as well. At best it will only delay the initial bloom flush a few weeks.

I know sometimes it's hard to imagine that far ahead when your vine looks good right now but truly, taking the time and effort to ensure the best possible long-term conditions for it to thrive will pay off in the long run.

I was just noodling around the Internet looking for general information on clematis to help me identify one of my best bloomers, and I came across this information from the Royal Horticultural Society, and remembered your question. For what it's worth, this is what they said about pruning group 2s: "Unless the plant already has three or four healthy stems growing from the base, all newly planted clematis should be pruned back hard the first spring after planting. Cut back to just above a strong pair of leaf buds about 30cm (1ft) above soil level. This will encourage multiple stems which can be trained in to supports to give a good coverage."