Interestingly enough, my 2.5i wagon on 87 knocks or pings alot. I changed over to 93 and it doesn't happen. When I am tight on money and go back to 87, the pings/knocks start again.
The gas station or company has no affect, it's the octane for my car that makes the difference and I haven't read much about others having the same issue.
With the A/C, I have to 93 or my car cries like a baby.