The service manual reads "Your computer uses 1066 MHz and 1333Mhz DDR3 unbuffered or registered ECC SDRAM memory" I'm old enough to have bought these new, and I'm pretty sure the Dell web page only ...

But no, it's not strictly needed for a NAS and the failure cases it covers are fairly small. The incremental cost of ECC in a DIY NAS is minimal, so it's something I've always opted for, but you can ...