As summer progresses several of the acers continue to produce some new shoots however, we would not normally need to prune any of this late new growth until the end of winter.
Occasionally we have performed the acer pruning in November or December without any problem; and in those years the winters turned out to be mild..........but we think if the winter turned out to be harshly cold then it would have been better to have waited to January or February to do the pruning!
What we do may not be what the horticultural books say but, it works for us!
Many of our acers are kept to a low mop-head shape, several are kept as small mainly branched-head standard trees and the occasional acer we prune to what we call a 'standard mop-head' by which we mean a small compact crown of branches like a 'lollipop' or 'mop-head' on an upright single stem of about one metre in length.
Despite being over 20 year old, many of the acers look much younger because of their size. Because they have a robust trunk and root system they look very healthy and produce an abundance of fresh foliage every spring, which brings such delight for us and our many visitors.
RHS advice on pruning Japanese acers: Acer palmatum is best pruned when fully dormant (November to early February), as maples bleed sap from pruning cuts at other times, weakening the tree. However, pruning is still best kept to a minimum as the most graceful shape comes from a tree that has been allowed to develop fairly naturally. As a result, just remove badly-placed or crossing shoots to encourage a good framework of branches to form. Where you do need to reduce height and width, follow long branches back to a side branch and pruning it out at this point. This is not necessary on prostrate-growing trees because they should be allowed to spread naturally to gain the best effect.