I've always had this rule I follow and I can't remember who passed it on or where I read up on it: when dispensing, to always match the refractive index for both eyes, irrespective of the Rx.

Today I was asked by a colleague, "but why?" And I couldn't find an optical reason why not (blonde moment..?) given this Rx:

R +5.00 /-1.50 x 38
L +0.75

The situation arised when the colleague was finding the best pricing solution, and was asking which lens to use. i.e., grind aspheric high index for the R eye, and stock aspheric CR39 in the L eye.

Aside from matching AR coats or extra edge thickness for nylon/rimless frames - are there any genuine optical reasons for matching indices? Perhaps matching base curves? Or was this one of those rules that created itself and became habitual with no merit?

-Memoir.

ps: A second 'rule' came into mind which I wouldn't mind collaborating with you all, "for single vision plus powers, if grind, grind both eyes"..?