Absolutely no reason to fill anything other than 87 octane unless you find ethanol-free gas.
If you can find ethanol-free gas within 10% of the regular gas price, it's generally worth it. Theoretically E10 is roughly 3% less energy, but in practice I find it to result in about 10% less mileage, so 10% is the price threshold I use. I suspect gas mileage suffers due to a shift in gear selection, with ethanol fuel resulting in lower gear choices more often, and thus more fuel consumption.
Further, E10 and other ethanol-blended gas are more susceptible to water condensation, leading to a greater potential of engine or fuel system damage.