The rule for oak trees is to prune before April 1 and after they drop their leaves in the fall. Oak wilt disease will enter easier on fresh wounds within this growth period, apparently, and this can cause the tree to die in a very short time. Pruning all trees just before the sap runs in the Spring will minimize the drying of the cambium layer (just under the bark), thus shortening the time that the wound will heal around the stub. But if doing many trees, this timing isn't practical, so during the winter is the best general rule.