First of all I would like to say a few thing about Heimerdinger as an champion, he is a brilliant mid/ap caster champion that needs some time to practice, and is all about POSITIONING.
You aren't the kind of champion which tries to run insade, kill everyone and run out. You are the kind of champion that creates a specific tipe of gameplay where you practicly take a role of artillery. You sit back and enjoy until your turrets do their part and engage with your other 2 skills to "CLEAN UP".

Masteries are same for all ranged AP casters, includin Heimerdinger. I take 21 points in offense for the AP and 9 points in utillity for mana regeneration and longer buff duration.

This is one of the rare builds in which I encourage you to leave the BLUE BUFF for your allyes. Mana won't be the problem. So won't the AP.