Dr Alex Shigo also included in a tree book that trees - in general - can be pruned any time of year.
Some people warn against pruning at certain segments of the year, but the significance of the reasons are dwarfed by the fact that tree services are not going to shut down for additional seasons of the year when they already have to deal with holiday and winter slow-down.
Especially since any hazard in highly insignificant.
In Oregon, we have pruned during every week, of every year, during heat, drought, wet, wind, cold, snow, etc., and never seen any problem on any tree as a result.
Other tree services and landscape specialists are seeing the same results.
That's in consideration of avoiding "heavy" pruning at sensitive seasons.
The only problem that we consistently run across in relation to season, is removing top growth on trees like Apple right before the heat season comes in. That results in sunburn since the sun shines hot on the bark before the new shoots and leaves can fill in and protect with shade.
Other parts of the country may see different results. But in Oregon, the trees themselves are the Phds, and natures own tree doctors, here, say that pruning works all year long - with success and health.
M.D. Vaden of Oregon