I think pruning it now would be just fine. I have been doing a lot of pruning lately.

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This post was edited by Easy69 on Sat, May 25, 13 at 0:54

I haven't grown Wisteria but I am a licensed arborist and I know that Wisteria is a very vigorous and hardy vine. In my opinion, you'd be fairly safe pruning a Wisteria vine at any time of year as long as you follow proper pruning techniques and don't overtrim it. By overtrim, I mean absolutely shear all of the foliage off. As long as you leave enough green for the plant to maintain it's photosynthesis process and you make clean cuts at bud nodes, any plant can be trimmed at any time.
If you really want to be careful about it, prune it in stages a couple weeks apart. Trim a bit, let it recover, then trim a bit more. Be sure to fertilize with a balanced, quality fertilizer to help the plant in it's recovery as well.
Hope that helps :)