Like Anobix said, it doesn't matter what spec. Just remember that you can cast spells from all three schools and use all levels of spells learned (level 1 frost bolt for example).
As a fire mage running through SM, I noticed (AND was told) I was drawing too much aggro from the tanks. So I started alternating between lvl 1 frost bolt and main scorch on the mob the tank had engaged 1st. That way, I could keep the target slowed during the entire fight and build up fire damage slowly by stacking the scorches AND keeping aggro away from me. Once the mob health was down to 25%, I fired up the main nuke and ended the fight. Move to next target. We discovered the fights were quicker and less healing was needed. I ran with a mostly pre-made group but even with PuG's it worked very well and was appreciated.
Playing this way makes the group want to go faster, as it seems they are mowing things down easily (which they are thanks to YOU). So, be sure to drink after every fight and even if they start another, don't jump in until you are at full mana or close to it if they need rescuing. I would also hang back next to the healer during a fight and if a mob was drawn to them, you can solo it or sheep it, keeping the healer healing and not fighting.
Yes, I do think doing tons of damage is important, but it is also important NOT to draw aggro and that is the line you walk as a mage in a group. We are a support unit not the tank (mostly). Using AoE's in a group will draw every mob to you and draw the ire of the entire group once you die, with time lost to rez or run. I can only answer regarding pre-50 instances, as I have not reached the raiding levels yet. I would assume, however, that our role doesn't change much.
So, in final answer to your main question: I agree with Anobix regarding spec (Doesn't matter), don't forget your role in a group and remember you can use all schools and all levels of spells learned.
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Thamage -- Lvl 52 Undead Mage - Duskwood
Hulkababy -- Lvl 22 Orc Hunter - Maelstrom