I have the same dilemma as the OP. Costco doesn't sell 89, and 91 at Costco is less expensive than 89 at the other dealers.
There is a noticeable difference between 87 and higher octane (at least with the winter formulas and in my '11). I've been alternating by tank, but can clearly see and feel the difference in performance and mpg.
So, to reiterate the OP, assuming I'm not throwing money down the drain because 91 at Costco is less than 89 elsewhere, what's the risk to the engine? I'm assuming the warranties (both original and Chrysler extended) would fully apply with higher octane use.