I would actually say in early spring would be best. right before the buds begin to swell and leaves start to emerge when the tree is still dormant and about to start becoming active soon. I have a few Jap. maple Bonsai's and this is when I always do any needed pruning and shaping.

If needed you could also probably do some pruning in mid summer after the new leaves have 'hardened off' and lost their soft delicate texture and look, and the tree seems to be in full swing. If this is the time of pruning though, I wouldn't do any major pruning or shaping as it might cause more stress doing it that time of year which can cause health problems.

So I think I'll wait until the buds are at least leaves.
I have the little seed things on some of them and they're beautiful!

Thx.

I prune anytime of the year. It won't get done otherwise. No harm so far.
Mike

Here is a link that might be useful: Fall foliage slideshow in my garden

The sap of maples becomes active early and pruning in spring will result in the trees 'bleeding' or frequently producing copious amounts of running sap at any pruning cut. This is typically of more concern to the one doing the pruning than it is harmful to the tree itself, although if pruning in wet spring weather, often encourages the entrance of fungal pathogens.

Ideally, the best time to prune Japanese maples is in winter before the sap starts to rise or in summer when the sap is not as actively moving. I tend to prune out deadwood in winter, as it it is most visible then and do any other major pruning in summer when the tree is in leaf and you can judge how best to thin or lighten the canopy.