Doesn't always seem to be the case any more, it's more like they look at the local market and just match the local price
I would have thought Tesco in particular would be selling their fuel a bit cheaper given all their recent publicity, but then their local forecourt is as busy as ever so I guess they don't see the need any more.
I don't use the Tesco 99 stuff that often, but mainly because it's usually so busy there when we go shopping that I can't be arsed waiting.
I do know my T5 hates their standard 95 unleaded, again I usually use super but when the end of the month looms she often gets a tank of "normal". The car feels seriously gutless on the Tesco 95, noticably worse than other brands.
Cheers,
John