If ZFS is anything like BTRFS, then I don't think ECC is super necessary. Yeah, it'll prevent problem from eventually cropping up when the RAM shits itself, but it's not like those issues compound silently in the night. When my ram started going a bit down the drain, (Flipping the occasional bit. Rare enough that the system still ran fine) I was hounded with checksum errors and any attempt to work with the data was met with error messages until I got new RAM. I don't think ECC is really all that important unless you're dealing with data worth thousands of dollars or more.