Form a "tree energy" standpoint, it is better to prune before the leaves are out. They do have to use starch reserves to develop leaves.
If you prune before the leaves are out, that starch is not spent, so no energy loss.
If you prune as soon as the leaves come out, the starch is used, but the leaves haven't had time to "earn their keep" by providing sugar back to the rest of the tree.
If you prune several weeks after the leaves have come out, they at least had time to contribute something back through photosynthesis before being removed.
Most of the time, it is not a big deal, but I am pretty sure that is why they say to avoid it.
I agree in general with these statements...one addition I would make is that heavy formative or reconstructive pruning would be better done in the summer.
Removing large amounts of wood in the spring (before leaf break) does to some extent mean removing stored energy reserves that are located in branches, twigs and behind buds. So the tree has to find enough energy to break buds, form leaves AND grow callus and woundwood after a spring pruning.
If the reconstructive pruning is done in the summer, energy production is ticking over smoothly, not on the steep upward demand after spring leaf break....do I make sense?
As for wound sealing, I have learned that in certain parts of the US there is concern for specific pathogens that can enter fresh wounds. Again I would say in general wound sealing is unneccessary EXCEPT where there is a defined issue with a specific pathogen. We use pruning paint on citrus trees because of a virus that is rampant...not sure it actually works though.
There are so many spores and creepies floating around in the air, they have landed as soon as your cut is made...putting a sealant on top of it just seals them in nice against the wood, moist, warm....
Properly placed cuts are the best option, allow the tree to utilize its inherent sealing and compartmentalizing strategies. Pruning at the right stages of tree growth, minimizing the need for large cuts...
I have observed many large pruning cuts and storm damage on trees over here, that from outward appearance you would think must be leading to spreading rot way down into the core of the tree...not always so!
I had a large Poinciana to take down,(too bad the tree had to come down to make way for construction) usually a tree associated with rot pockets and internal decay. This one had a large (24") old wound that had not closed...soft decayed wood on the outside. When I had got it all down I dissected the area and the decay had precisely followed the branch tissue down to the origin of the bud in the trunk, a classic 'cone' shaped compartmentalized area! It also had an old cavity with concrete in it...I expected a long column of decay leading down...not so, sound wood at the base! These wounds were at least 20 years old, and the decay had not spread beyond the branch cone!
A tamarind that had suffered greatly in a hurricane five years ago, shattered ends, multiple resprouts...the dieback was minimal, the extent of decay was way less than I would have expected...callus/woundwood was strong. Trees can outfox you..amazing stuff!
Then this week, an avocado, split branch, loads of internal decay, weak points failed in high winds we had for two days...but it had never been taken care of properly...the badly placed cuts and neglected pruning needs were what led to the problems!
I will qualify the above by saying that we do not have the range of nasty tree decaying fungi and destuctive insects that you all have...so maybe we luck out in that regard!
Happy Easter all!