Most of the preview builds definitely benefit from a hard reset. I've seen this at least twice.
For the final release of W10M, if you're upgrading from W8.1, a hard reset should not be necessary, at least that's the theory. Whether that's how it plays out in practice we simply can not know until the final release is actually available to us.
If you're upgrading to the final release from an insider preview build, then I'd still recommend a hard reset however, just to be sure you're not bringing along anything funky from a buggy or broken insider preview build.