Yeah I do about a tank a week and use premium (US 92-93 octane), so it matters as it's a lot more expensive than other areas. I've noticed around Chicago, premium goes for $1-1.20 more per gallon, but if I travel to other areas like Indianapolis, St Louis, etc, its only maybe $.50 more (though it's been a while since I've been in Indy).
Anyway, with a 19gal tank, that's a big amount over time. If I save $.25/gal, that might only be $5/fill up, but in a month it's $20+. I also seek the better 93 octane since technically 91-while "premium" doesn't give me as good of mileage or power either.