Commonly the best time to prune is in late winter, early spring, when the coldest time has just passed and the tree starts opening its buds. Even though some species i.e. maple and dogwood, will bleed sap, it will not could not damage them.
It is always better to avoid tree pruning during hot, dry or the coldest periods of the year. If you missed the dormant period and are calling the tree service when the tree just got leaves on, best choice is to let it fully develop its leaves first. And once they recoup the energy they used for it – go ahead and make them healthy & pretty with tree pruning!
