Some very talented grafters are able to make a reasonable percentage of grafts take even after the rootstock has leafed out. The reason that most people whip and tongue graft right before the stock is leafing out is because the green push itself helps the plant to succeed in growing into the selected scion.

Speaking as a very average grafter, I would rather concentrate my time and energy when the grafting is more likely to be successful. I might get 80% on a good year as the tree pushes out, but I'm more likely to get 10-20% take on a summer whip and tongue. It's not worth my time, my scions, or my energy. I have lots of other things to do during the summer other than unsuccessful grafting.

Some grafters are incredibly precise and able to get high rates during the summer. I am not. Nor are most grafters.
John S
PDX OR