In the mild-climate part of the Pacific Northwest, if a birch is very strong and in a vigor-oriented potting setup (i.e. not a super shallow tight bonsai pot), then that birch might get pinched several times in the early spring, then cut back strongly at the end of May/June (perhaps w/ some leaf reduction), then pinched again through the rest of the summer to promote ramification or to knock back suckers and watersprouts. Then it may sit to grow from early August until leafdrop at which time it is worked again, except with the constraint of no big cuts (should have done those in June). My teacher's answer in a mild climate for a strong birch might be something like: Work birch throughout the entire growing season, know which operations are appropriate for which growth states. This is not followed blindly since to do some of these operations (leaf reduction, etc), you need to read the extensions/shoots for vigor and make a call based on that.
In my experience so far, dieback in things like birch, but more dramatically thingsl like willow, poplar, cottonwood, etc is one way or another related to disruption of water transport to the tip of a given shoot or branch. Examples:
1: Wire aggressively in zone 6 or 5 at leafdrop time and maybe everything works out due to good winter shelter or a strong tree, or maybe it wakes up to some branches that couldn't heal before dormancy set in, and withered over winter.
2: Ignore a strong emerging sucker near the base of the tree, or similarly a strong water sprout near the armpit of some critical trunk-branch (or branch-subbranch) junction, and maybe it overpowers other nearby growth to the point of weakening that other growth and causes dieback elsewhere in the tree. Ryan Neil has characterized the sucker/sprout case as a "short circuit to the roots" that disrupts water transport (for other nearby growth, but not the sucker itself) in some way.
3: Pinch too late in the year and maybe the cut site doesn't recover well and disrupts more of the shoot than I intended.
My deciduous teacher helped me "tame" black cottonwood to be much less statistical/probabilistic exercise by paying attention to the suckers/water sprouts, and also we're learning that cottonwoods don't love being asked to heal from aggressive wiring while needing to move a lot of water. In your situation, I would be tempted to say that the "work birch throughout growing season" regimen as the theoretical ideal, but that in real life in colder zones and shorter seasons, my approach would have to be a conservative subset. Not pinching as far into summer (maybe stopping in early July and letting extensions run), perhaps moving wiring from fall to spring, etc.
