Recently I thought it would be useful to register my left thumbprint in addition to my right one (I am right-handed), as I sometimes need to turn on my iphone with my left hand.
I did not expect that every single app that requires my thumbprint to log in would then require me to re-register my thumbprint from scratch simply because my biometric information has changed, even though my right thumbprint is still registered. For some apps this was fairly simple, but in other stricter apps this is quite a hassle. I would not have bothered to register my left thumbprint if I knew that I needed to do all this.
So if you're thinking about registering your non-dominant hand on iphone biometrics in addition to your dominant hand, please bear this in mind before you do.