It really depends on your play style, whether you pvp and if you have mastery amulets. A lot of Balance go with Myth as secondary. I did Death for mine because I had a mastery amulet from another wizard, and I planned on training death to feint anyways. It came in handy against Balance bosses.
seethe42 has a point; it really depends on how you play the game.
my sorcerer was my first wizard. i had no idea what i was doing at the time, and i picked fire as a secondary; i ended buying back all of her points at level 80.
imho, the only secondary schools you really need are sun and star, available at level 52+.
a myth mastery amulet (for shatter) is pretty much a requirement for balance to pvp, since we can't wand off shields or weakness; death (to feint) and ice (to tower) are good choices as well.
