There are a handful of special purpose digital servos that will get overworked on 6v. In helicopters we run a voltage step down to the tail rotor servos because the gyros update the servo at 270hz vice the usual 50hz they get from a receiver. RPM governors that run in high frame rate are the same way and they will wreck a servo that isn't rated for 6v (5 cell)

Using a regulated 6v system powered by a lipo it isn't a problem even for servos rated only 4.8 (high frame rate stuff is the exeception)