my personal experience is that I get a marginally better running engine when I run premium as opposed to middle grade. Though I will note that when I run premium its from mobil so its not an ethanol blend. But the times I've bought middle grade its from the co-op which is a 10% blend. Like I said, the difference is nominal, but its enough for me to justify the cost. I agree with FNquickF2.
If the difference between grades is 10 cents and I'm only buying 3 gallons, the cost difference is pretty miniscule. I'd rather spend the two extra quarters for my marginally better performance than deal with anything that may affect the longevity of my engine.

 

I second elmer's notion. I run 91 octane (premium in Ca) just because of predetonation. I'm pretty sure our motors have high compression ratios which require higher octane gas than 87 or 89. I mean, if you were using a 1990 bmw 325i with a 2.5 liter engine with a compression ratio of 8:5:1, using 87 octane is your best bet (My car). But other than that, bikes = high compression = high octane = cleaner burn = less probability of predetonation = more reliable.

Cheers.
-Jasper