I have two 4-5 year old clematis jackmanii that climb 6 foot trellises that frame a window. The past two years, I've pruned them down to 10-12 inches in late winter, and they end up growing to the tops of the trellises and bloom spectacularly. I forgot to do that this year and the dormant long stems have begun budding near the tops. Is it too late to cut stems back to the usual 10-12 inches or will I be better off leaving them as is at this stage?

Any advise would be welcome. Thank you