Touch ID best practices
Apple does not provide any guidance as to which fingers you should set up. It is purely a matter of personal preference as to which fingers you use based on the ways you hold your device when unlocking.

I suggest adding fingers you consistently use when trying to unlock your device to Touch ID, and removing any fingers you don't use often. In case you didn't label your Touch ID fingerprints, you can see which fingerprint corresponds to which row in Settings by holding your finger on Touch ID when in the Settings page.