In the north- which you are in- the reason for waiting for late winter to early spring is so that the coldest of the weather has passed. When pruning, you prune back to leave a certain target number of buds- based on winter damage to the buds. If you prune to a target of say 50 buds you can't change that. Let's then say you get a severe winter and 50 % of your buds are killed, you will only only have 25 left for next year. Not only does that make the remaining buds which grow into shoots too vigorous, but your potential crop will only be 50%.
Up here, wait until late winter/early spring.
I would start towards the end of the vine and try to sort out/prune back. Idealy you want one or two trunks and depending on the trellis you make you will want to keep two to four canes for supplying shoots for grapes. Remember each bud left on the bronze colored wood grows into a shoot and will have about two to 3 clusters on each shoot. If you can leave four canes with each 12 nodes, that would give you 48 shoots which will be plenty. There is a lot more to it than this, but you need to get abasic structure going before gettingtoo fancy.
Last edited by grapeman; 11-22-2010 at 10:27 PM.