I think what they mean is make sure one side isn't too prominent volume wise or frequency wise (compared to the other). If you've ever listened to a song with a bad stereo balance, you'll feel it in one of your ears. It almost will feel like an OCD thing, if that's a familiar feeling. But everybody except the most open of listeners will hear that and think hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm.
I, for example, wouldn't put a super high pitched whistle hard left if there was nothing to balance it out on the right. And I try to make sure whatever is balancing it isn't on the complete other side of the frequency spectrum, because the high whistle still tends to bother me. But I am a perfectionist and not a very good mixer to start with, so that's just me.