Spring is a good time to do it, just before new growth starts. But it all depends what shape you wish to enjoy them in for the winter. Whichever you find the most attractive and pleasing. Pruning them now will not do them any harm.

Thanx Johnathan 4 that :) if i trim/prune them now wont the Frost make the Tips go brown? :/

I hadn't thought of that.

i think i'd leave it until spring so you can adjust if there has been any winter damage

Thanx Seab :)