True, some trees such as maples and birches will "bleed" or lose sap from pruning cuts made early in the spring. This bleeding does not hurt the tree, and the loss of sap is inconsequential. With a few exceptions, most routine pruning can be done anytime of year. The worst time is just as the tree has leafed out in the spring. The best time is when the tree is dormant. To maximize flowering for the following year, prune just after bloom this year.