I realized recently that we aren't all on the same page with backporting.
We currently only document the following:
"Typically the fix for an issue that is affecting supported releases lands
on the master branch and is then backported to the release branch(es). In
rare cases, the fix might directly go into a release branch without landing
on master (e.g., fix / issue is not applicable to master)." 
This leaves room for interpretation about what lies outside of "typical".
Here's the simplest way I can explain what I stick to, and I'd like to hear
what others have in mind:
* By default, bug fixes at any level should be backported to existing
release branches if it affects those releases. Especially important:
crashes, bugs in non-experimental features.
* Exceptional cases that can omit backporting: difficult to backport fixes
(especially if the bugs are deemed of low priority), bugs in experimental
* Exceptional non-bug cases that can be backported: performance
I realize that there is a ton of subtlety here (even in terms of which
things are defined as bugs). But I hope we can lay down a policy that gives
everyone the right mindset for common cases and then discuss corner cases
on-demand in the future.