Performing a factory reset before upgrading to Windows 10 Mobile has been reported by numerous users to be a good option, since it supposedly leads to better performance and stability. However, some users also reported that performing a factory reset is no longer necessary given the stability of the most recent (Slow Ring) builds.
In my opinion, if there's any advantage of performing a hard reset first before upgrading, it is in it being a matter of precaution. On the other hand, the only advantage of not performing a hard reset first is that not doing so saves time (from performing the hard reset itself and in restoring all your files and settings back which combined could take hours).