Depends on whether or not you'd be cutting off your buds.

I'd pinch it back as it is first growing rather than cutting it off at the top, it will just grow again from the top.

Does your Purple Clematis just bloom on the top or all over?...Jeanne

The "extra" length of the clem cascading over the top is part of the beauty of a clem, in my opinion. I would never trim it off.

Kate

I believe it flowers all over. I never thought about pinching it-too late now but I might next year. This is a new structure so I should see what it looks like when it flowers and spills over. Thank you all so much!