I can perhaps understand not using thumbs and index fingers as these are the ones we probably use the most for precision handling of things, so could cause the most hassle if we ever did do any damage to the nerves.
I've never understood about using the sides and not the pads, as it's the sides that hurt
me the most...
Being left handed I nearly always use my right hand fingers for pricking and certainly my little finger is the most prolific blood wise...and possibly the one I use least of all for everything else.
Robbity