From the Royal Horticultural Society website:
"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."
Source:
http://apps.rhs.org.u...aspx?pid=126#section2
Chris