Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my take on Teemo. I have tried standard DPS/carry and end game he lacks the extra umph the get the job done. However by picking up items that boost AP it gives his blinding dart more a sting, his toxic shot a slight damager boost and his shrooms become that much more deadlier.
I arrived at this build after much trial/error and upon reading Blue's extensive teemo guide on league craft.
Below is some well sound advice on boot choice.
Berserker's Greaves is a common choice. I do NOT like them. Get your attack speed elsewhere. Get more beneficial effects from your boots, whether it be the increased speed of Boots of Swiftness, or the CC reduction and magic resist of Mercury's Treads. Even Sorcerer's Shoes (for AP Teemo) or Ninja Tabi are better. I don't really like Boots of Mobility either, but they're the boots that almost nobody takes, anyway.
Don't take Greaves.