Usually more damage is better than faster attacks, but Teemo deals both physical and magical damage on his attacks. I think he's better off with AS marks because of this. Just build like TheRainman and you're good to go.
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I think it depends on what build you're using. I build Teemo with a lot of AS and magic damage, so I find the mPen marks compliment my build better. Teemo synergizes very well with a lot of AS items (Berserker's Greaves, Madred's Bloodrazor, Malady, Nashor's Tooth etc.), so the boost from runes isn't going to be as helpful as it would on a stronger AD build with less attack speed.
Also, runes, especially flat ones, are generally more important early game than they are late game. Early game, Teemo's poison does his harrass for him, allowing him to deal significant damage over time with only one auto attack. mPen marks significantly increase the damage from poison, while AS marks will not. Between 1-attack-harrassing and last-hitting, I don't find myself in need of AS before I get mid-game items, which give me all the AS I need.
Hope that helps.
Also, runes, especially flat ones, are generally more important early game than they are late game. Early game, Teemo's poison does his harrass for him, allowing him to deal significant damage over time with only one auto attack. mPen marks significantly increase the damage from poison, while AS marks will not. Between 1-attack-harrassing and last-hitting, I don't find myself in need of AS before I get mid-game items, which give me all the AS I need.
Hope that helps.

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Yea, but you'll be grabbing the MrP mastery anyway. That reduced the effectiveness of MrP marks by 15%. I think AS marks is the way to go. It also helps with lasthitting.
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I main Teemo, and I use AS reds instead of M/A Pen ones because I think that more hits > less hits that deal more damage. I build my Teemo with AS+AD or AS+AP items depending on my mood.
Am I doing it terribly wrong?
Cheers