If you've got a lot of desert, Water is huge in the early game. Air is great for offensive arcanists (Maelstrom = the hotness, Air Elemental is very nice for taking out pesky ships without even needing a navy). Body, Mind, Spirit and Enchantment are all great perks, but not at all essential. If there's a lot of mana on the map, Metamagic is freaking awesome... but a waste if there's not much.
That about covers it. Obviously if you want a Tower of Mastery victory you're going to need one of every mana.
So... for most civs... ~three raw nodes is plenty, unless you want a magic focused game.
Oh yeah! Keep in mind that if you vassalize your rivals, you can force them to hand over all their mana for free. So, it's usually not actually beneficial to go for the complete kill when you can make them capitulate (although if you're already winning/have enough mana/whatever, you may want to kill them anyway, technically beneficial or not).