Yeah UUID would be the way to go. That it doesn't do that anyway when you address them as /dev/sdX, like most of the examples online do, seems odd. I was pretty surprised to find it was tied to the system-ordering if you add them the user-friendly way—you've gotta do the translation to UUID for it, when adding the disk.
Since restoring a disk that had changed its system-ordered address meant re-silvering the whole damn thing (even though the data was already the same?) I was reluctant to do that a third time to fix it by changing to UUID. For now I just don't touch the disks. Eventually I'll replace the externals with internals and then I'll take care of it.
EDIT: that's exactly what I mean—examples strongly favor /dev/sdX (or similar) but in fact you want to use UUID and the zfs command line tools don't warn you, let alone default to transparently using UUID instead unless told otherwise, either of which would be a clear improvement.
There are some easy symlinks under /dev/disk/ you can use.
I ten to use either by-uuid/ or by-label/ myself.