Just to add a little confusion:
Peter Adams' in "BONSAI with Japanese Maples":......structural and heavy pruning is best done in August, trees are at a plateau in their growth ......There is enough time for some healing before the frosts arrive."
Vertrees: "....major pruning should be done during the dormant season from late November to early January, well before the sap starts rising...", then "....corrective pruning can be done any time of the year except when the sap is rising..."
America Horticultural Society Pruning & Training Handbook: Prune in Winter when fully dormant: maples bleed sap profusely at other times. Small cuts may safely be made in late summer/early autumn"
Sunset Pruning Handbook: " Avoid pruning is Spring just before and during time of active growth: most bleed sap profusely then"
Sunset Western Garden Book"To minimize sap bleed, make any cuts in summer or early fall in mild-winter areas, or from summer to the end of January where temperatures remain below freezing"
James Harris ( 'The Gardener's Guide to Growing Maples'):" Pruning is best carried out when the sap is falling in late Summer and early Autumn, light pruning can be carried out in the Spring"
George Brown (in The Pruning of Trees, Shrubs and Conifers) says: "....carry out any pruning which is necessary in the late summer or early autumn"
See why I'm confused?

Granted a lot of this is climate based and varies but it seems like they all agree that major pruning in the spring is unwise.