There is no real bad time to prune trees, with a couple of exceptions. In areas where Sudden Oak Death is an issue (not here, fortunately), they and other susceptible trees should not be pruned during times the fungus responsible is active, so it can not get into the fresh wounds. Other than that, the only other bad time to prune trees is right when the buds are coming out. Trees are trying to push out and grow new leaves, but with no existing leaves to produce food for this process, the trees rely on stored energy.
If you do need to prune a couple of branches then, however, I still wouldn’t worry too much.
I bring this all up in the context of sap because the sap is indeed flowing well this year. Maple syrup producers must be happy this year, warm sunny days, cold nights, and problems in Canada mean a bumper crop this year. We’ve pretty much stopped pruning trees on campus, not because it is a bad time to prune, but because of the sap. It’s a little disconcerting for people walking by to see a tree “bleeding” profusely from an open wound. I remember pruning a large limb off a birch last year, and getting many phone calls from the offices nearby concerned about the literally gallons of sap flowing out of the tree.
I bring all this up only because I’ve got a great link to share. The Proctor Maple Research Laboratoryat UVM (my old Alma mater) has some Sugar Maples hard-wiredto the Internet. Graphs of current Air and Branch temperature, sap flow, and trunk temperature in real time streaming live to the Internet.