We wait until all the leaves have fallen for a few chops, and even then don't bother to give our's a haircut until the Spring!
It's over 200 years old, and has a trunk about 18" diameter, and while we tend to give it a hard time every year, it comes back and goes mad, such that we have to have a chap around to get it out of the roof, and as Mrs Growster has forbidden me to go up a long ladder ever again, (I fell off pruning the thing once), I have a superb lopper which extends enough to get it under control now...
They do take their time to start flowering though, and suggest that I'd not worry about any pruning until the Spring, just before the buds form.