This is really just tired old wisdom handed down from the days of wide inflexible chains. Modern 8-speed+ chains can cover the whole cassette with ease. Additional wear from the dreaded cross chaining is never going to be significant when the cogs already have profiled teeth. It's always funny to read the hand wringing from MTB riders about this when they don't even bat an eye at the horrible inefficiencies of 20psi fat tires.
