Dormant or late winter pruning is recommended because the cold air and freezing night time temperatures suppress the growth of pathogens. Dead wood should be pruned away as soon as it is noticed and pruning at other times has some risk associated with it but maples are not like oaks when there are times they should not be pruned. Don’t take off more than 1/3 of the tree because the tree needs about 2/3 res of its mass to make enough food to thrive.