Sorry for necroposting, but i had to respond to this:
But I remember blizzard normalized the mana cost of lower ranks or something at some point, so I am not sure this actually saves any mana.
Yes, Blizz made some changes to downranked spells in TBC, and finally killed it in WotLK, but we're talking vanilla. I've found some info that they nerfed efficiency of sub lvl 20 skills, but could not find info when exactly (pre, or post 1.12.1), but it really doesn't matter for mages, since you're casting rank 1 Frostbolt/Firebolt etc. not for its damage, but for the effect that comes with the spell (slow, DoT etc.).