I did a search and most answers were pointed in one direction but not all. So...
Where I live there is a Kum & Go with 87 Octane, 89 Octane with 10% ethanol, and 91 Octane. The 89 Octane that is 10% ethanol is the cheapest. Is it okay to run that in my 06 GT?
BTW, next year I'll have a tune and will be running the 91.

 

Yes, that will be fine. Your owner's manual tells you that up to a certain percent (I forget what it is...but it is well above 10%) ethanol is OK.