I have heard that from mechanics as well, but found no real evidence to substantiate it. In an article on gasoline from CNN here is an excerpt.
"Typically, within the same brand of gasoline, between regular and premium, "the difference is really just octane," confirms Prentiss Searles, Marketing Issues Manager at the American Petroleum Institute. To call a fuel regular, midgrade, or premium, Searles explains, only relates to octane; there's no other requirement that assures greater purity, more detergents, or better additives.
Nielsen says that the emphasis on detergents and additives for premium fuels is also misleading because in most fuels you have plenty of detergents to keep modern computer-controlled engines clean. "Unleaded is just fine," he insists."
Some brands do claim to put in more or better detergents, but apparently it goes into all grades. Then again when you are only buying 2 or 2 1/2 gallons the cost is not appreciablle for peace of mind.