Should I use ECC RAM for my non-critical NAS?

Probably not. For non-critical data like media libraries, game saves, or old documents, the risk of a bit flip causing real damage is tiny compared to the cost premium of ECC. ECC RAM costs more and limits hardware choices to server-grade components. ZFS’s checksumming will catch most corruption anyway, and if a cosmic ray flips a bit in a movie file, you’ll never notice. Spend the money on more storage or a good UPS instead.