Well I asked a similar question here a while ago so I'll repeat what I was told.Rootprune when the plant is 'Quiescent'...when the plant has reached it's chill requirements for the year but not woken up yet.
So sometime before the buds start to swell....I'm guessing about 3-4weeks,(maybe someone here can be a bit more accurate)...for me that would be up until about mid-March.Make sure the danger of the pot freezing has passed though.I do mine in autumn because I can get away with it but for zone 6 this is the best time.

Daniss, I have done many and always try to get em done before spring activity starts. I think with pot grown you can almost do them anytime but i dont once spring push is here, or when it is terribly frozen so i can give them a good drink in their new setting.

Here is an excerpt from an article by Tricia from Essence of the Tree:

"Timing is probably the single most important criteria to a successful root pruning. During dormancy, about 2 to 3 weeks prior to bud break is the ideal time. During this period there is no active root growth and no demand on the root system from the tree. If you are not sure when emergence is in your area, watch for a slight swelling and shininess of the buds."

This is from the newly arrived Spring 2012 catalog. If you're not on their mailing list, I highly recommend signing up. The catalog is beautiful with color pictures of diverse maples and also helpful articles like the one quoted from above. I've never ordered from them, though since they seem to be a little on the pricey side. Nice of them to keep sending me their lovely catalog year after year.

Here is a link that might be useful: Essence of the Tree website

After you prune them, do you place them back in dormancy? In my case, it's unheated garage? Or do you need to protect from freezing (which would then become my basement) for a few weeks? First experiment, as the maples I got, their roots were so girdled and looked too brown (Shouldn't they be white?)... I may have killed them all...I studied a bonsai book, but I may have lost a lot of money.

amy

Amy, You didnt remove them from dormancy just because you root pruned for a 'pot up' or 'repot'. Treat them the same as if you hadnt root pruned. Unheated garage sounds good. Dont let em completly dry out,but they do not need to be soggy. Then go through your normal steps to get them located outside come spring.

THanks Alley cat, I'm still scared... first timer, but we'll see. Thanks...

amy

I have many maples growing as bonsai. I root prune immediately before or at the onset of bud movement in the spring.

After the root work, the roots should be protected from freezing temperatures (unheated garage), as should the rest of the plant if bud initiation has commenced. Once growth has commenced, you can destroy the appearance of your tree if you don't give it enough light. Even if you have to move the tree(s) outdoors and back indoors as temperatures dictate, light is important to avoid sections of weak growth and long internodes that will eventually become a permanent part of the plant's composition unless pruned off later.

{{gwi:3257}}

Al

Okay, now they'll go back to basement until this cold snap leaves. These poor guys, it will be a miracle if they survive. Thanks everyone...

If there is no evidence of bud movement yet, the basement isn't a good place for them. You should keep them where temps are above freezing but below 42-45* if possible.

Al