Have no fear of pruning your N. Spy right now. A vigorous old tree can be pruned just about any time that the weather isn't going to drop below -25 degree F. within a couple of weeks and I don't know if that would hurt it- it's just the critical cut off according to the literature. I have done major pruning on many varieties of old apple trees where the temps have fallen below -20 shortly after with no cambium damage around cuts. As far as summer pruning, it is only a problem with young trees that you are trying to grow as fast as possible or trees with inadequate vigor. Summer pruning is more de-invigorating than dormant pruning which can be a good thing.
I have found that even major pruning done at this time is actually safer than dorrnant pruning as far as preventing scorched bark on upper scaffolds. I believe this is because there is so much leaf development at this time that adequate sap gets pulled through the cambium to keep temps down even when a lot of wood is removed.
[NAFEX] pruning a Northern Spy in August,
Alan Haigh, 08/10/2009