From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1897-256954
Question: I want to trim the holly because it's getting tall. But each
time I look it has a new branch. when can I cut it down?
Answer: If just a single branch needs to be removed, you can do it any
time. If the entire plant needs a trimming, late winter or early
spring is the preferred time, with follow up done up until mid summer.
Pruning tends to stimulate growth and if you prune in mid fall the new
growth will not have time to harden before winter sets in. As winter
approaches the growth rate will slow, so the problem shouldn't get
much worse.