Nice point about flying opponents - but I should mention that I kept both Fireball and MM maxed out before I spent points in Destruction, so I benefited from fireball before FoV.
The maths that I did indicated that spending two skill-points across MM and Fireball was more than twice as good (damage wise) as spending a single skill-point on Destruction, therefore you should max out at least two damage spells (MM + Fireball/Magic Blast) before spending points on increasing damage (Destruction). Ultimately you should max out them all, as well as 'way of the battlemage', but leave that until FoV when there are skill points to burn...
Obviously, the first point you spend on destruction gives you much larger bonuses than any future points, so spend that first point as soon as you can, but leave the later points until you've maxed out BOTH your favourite damage spells.
But don't take my advice too seriously... I like to think I know what I'm talking about, but that only applies to my own playing style and my definition of a 'Mage' build. DKS is flexible, and just because my personal build worked well (first time through) doesn't mean other builds won't work just as well.
