I used all my fingers for a single fingerprint scan, and it actually works! I would still recommend adding an extra fingerprint scan for one single finger.
Recording more than one fingerprint just insures that, absent malfunction of the fingerprint scanner, if one fingerprint fails to work, one of the others will. In my case, I have been known to use a different finger, depending on whether I am holding the phone, or unlocking it while it's on a flat surface and which side of me the phone is lying. Also, sometimes your finger is wet or otherwise hard to read. There are times I am wearing a bandage or finger guard on my "unlock" finger.
Having a fingerprint of a trusted 3rd party would allow access to your phone if you are incapacitated or expired.
Absent any strange "bug" using this I find it helpful to be able to use any finger on either hand to unlock the device. It never occurred to me to try this. I actually tried a dozen times to register my right thumb so I could use in landscape mode, never could. I registered fingers (all 5) and thumb works with no problems, go figure.....
