The instability problem I'm talking about isn't about errors, blue screens or crashing. It's about micro-errors that minor instabilities introduce into your OS file system and non-OS file data like files for games, pictures, movies (Especially if they've been encoded to a different format), documents and backup data.
Obviously it's your system and if you're happy with it, then great. But if or when you start seeing unexplainable issues with programs, files or the operating system, you'll understand why full stability is a major concern. And if you are running the memory at any speed faster than the default 2133mhz, regardless of whether the CPU is overclocked or not, it is ALWAYS a good idea to do at least three passes of Memtest86 AND an 8 hour run of Prime Blend mode.
My current 6700k system with OC memory (And anything over 2400mhz is considered OC on DDR4) was fine for 7.5 hours in Blend, and errored out. I had to up voltage by .020v on memory and raise both Trcp and Trd back up by one point to pass 8 hours. If I had tested for less time, I would not have caught that and I'd have thought everything was fine when it wasn't. A stable system should be able to run any stability check practically indefinitely but certainly longer than 8 hours. 8 hours is just a commonly accepted timeframe suggested that it's unlikely and that in testing it's rarely been seen to still error if it lasted that long. Your system though, I'm good with whatever you choose to do.