I think it depends on how you deploy OpenWRT instances, as what roles, and how much time you would like to devote to this sort of activities. So, it's pretty personal.
I have a preference for the rolling update nature of the master branch (snapshot builds) over the backport-and-bugfix-only style of the latest stable branch. I am kind of OpenWRT enthusiastic user and tester, have a usable buildroot, and like the latest and greatest things with tons of fixes and enhancements, and allow certain unexpected down time as I use OpenWRT in my homelab environment.
Basically I am doing a little more than a typical end-user would do.
This is the deciding factor.
It's ok to use the latest stable release, if nothing unexpected happens, just deploy and forget about it, except when there is a new point release. More so when you deploy it as a self-contained kvm guest instance.
Running OpenWRT as lxc instance, that's another story. We are running OpenWRT codes on non-OpenWRT host Kernel, venturing into an area where there is limited support and user base. Thanks to the devs, OpenWRT is shaped to run fine as lxc guest with minor-to-none modifications. Nevertheless, regarding combinations of particular host distro other than OpenWRT and OpenWRT release as guest, related tests are likely not performed. In this sense, stable releases are no longer considered stable. Users of lxc instances are pretty much on their own. If you only use userspace programs, have few interactions with kernel modules, any builds would be fine. However, as you are using SQM or the like, then you may have to take kernel compatibility into account. Please do note, OpenWRT 19.07 is using Kernel 4.14, utilizing an out-of-tree code of CAKE. In this case, I won't bother to evaluate the impact on stablity as lxc instance, just simply avoid builds with older kernels.
I would like to keep the difference of the major version of kernels, between the one OpenWRT is built against, and the one the host is using, as little as possible, even if their patches do differ.
Current OpenWRT x86_64 master branch uses Kernel 5.4, so does Proxmox VE 6.2. This makes a natural choice for me.
When doing building and test myself, reading through commit messages is quite important for me.
First, I can see what have been changed and whether it's worthy to make certain snapshot build and then deploy.
Second, if something goes wrong, I can quickly git-checkout, identify the problems, make workarounds, or revert certain commits, and file reports if I have to.
From my own experiences, most of the gotchas are non-fatal, and can be easily fixed.
Working with master branch is easier for me, for many issues getting reported and solved upstream as time goes by. Then I don't have to work on something already fixed upstream. On the other hand, certain bugfix may or may not make it to the stable branch. Users doing the backport themselves completely defeats the purpose of using the stable release.
Finally, lxc instances are cheap. Switching is easy. This is the very reason I wrote Oplxc4pve script for easier deployment and instances switching. It's not hard to pick a "stable" snapshot build as my one of the waypoints for my ongoing journey.