I wish that were true; more discussion at Hiding Apple’s Big Sur Upgrade Badges - #25 by FlaSheridn. I also wish I had a good rule of thumb; I used to upgrade to the last minor version of the previous major OS release, but Goto Fail meant that all of iOS 6, and 7.x before 7.0.6, were presumably radically less safe than iOS 5. (Similarly MacOS 10.9 and 10.9.1.) Balancing new security knowledge by Apple against decreasing code and testing quality, not to mention new security knowledge by evildoers, is a hard problem.