Most trees are good to prune in dormancy. Some still say it is the best time.
Trees with lots of deadwood are easier to clean in leaf, because the dead is easier to see.
A small UofMinn study showed that there was a higher rate of wilt mortality in oaks pruned in dormancy then those pruned in dry summer. The thought is that the wounds can start to close or dry up/compartmentalize before the pathogen contacts the wound.
Some conifers will pitch out (bleed) more in the spring time, and are best done in dry summer when sap is moveing less.
Birch will have a simmilar timing effect.
To sum it up, it depends on species and client expectations.