Exactly what I was thinking. A slightly corrupt filesystem is likely to become a seriously corrupt filesystem and then a heap of completely useless bytes, if no-one gets to know about the corruption while it is too minor to be screamingly obvious.
Especially with a new filesystem like btrfs it would make sense to combine backup with fsck. Do backup, do btrfsck, if filesystem structure checks AOK allow it to continue being used, and recycle the oldest of your backups.