The manual just states that the effects of premium will be most noticeable in hot weather or when towing. No requirements to use it. I have not had any issues w/ 87.
One note, though, that I found in the manual -- it requires at least 87 octane, even at altitude. Here in Utah, our regular grade is 85 due to altitude. I ran 85 the first few tanks and didn't notice a difference when I switched to 87, but that's what I've been running the last few months.
I love my Ecoboost. Tows great, drives great, better mpg than my Chevy 5.3, with noticeably more power to boot. I'm a fan.