I recall reading somewhere that if you are to use premium petrol, you have to use it all the time to note any kind of significant difference, and that using is every so many tank fulls is pointless... a bit like wanting to lose weight, but only dieting on Tuesdays!
Don't know whether or not there is any truth in this: over my years of car ownership, covering 20k-30k per year, I have only ever used standard supermarket petrol in all my cars and neither them nor I seem to have fared badly for doing so.