Fwiw, I occasionally use most of the cassette in granny, but that is on extended climbs where the grade varies and a shift to the middle ring wouldn't last long. When the terrain is rolling, I shift back up as soon as possible. Same rule applies to granny as to big ring. Just avoid the little cog and its buddy, and if you do occasionally go up there, don't worry too much about it.
I see that "occasionally" is OK. The problem is what is the rule to follow so you know what to do only "occasionally"!