That is indeed a good question. Is this something RAID is bothered with at levels 0 and 1? I think in this case it's the job of the filesystem to care for that. But you should probably let the periodic task run that does scrubbing like once per week. You could also experience other issues than just bitrot. For example bad sectors and one of the hdds slowly degrading.
In the end I don't think a RAID1 can do much about bitrot and other RAID woes. There are no checksums or anything to correct for that. You'd probably need some other technology for that. But it's probably the same for a ZFS mirror. And everything better than that needs mote than 2 hdds.