Thinking back on it some more, I remember that around the same time as Dolphins & Friends, the show's lineup of grocery items became rather overpopulated with medicine and cosmetic products, which because of their generally high prices, could make certain games very easy (Hi-Lo) and others very hard (Check Out).
Earlier on...sometime in the mid to late '90s...there was a diet product called Pounds Off! that made lots of appearances, and in the description, Rod would always mention that he used it. This was during the era when Bob would often ask Rod how his diet was going.
On your second question, I'm in the middle of the road. I always liked how the show rewarded regular viewers who paid close attention by repeating certain products very frequently (in turn, giving people an incentive to watch more often). But I also think it's good for most of the games to demand that contestants have a general idea of what things cost (that's what the show is supposed to be about, after all). So I'd split it down the middle and say I like for grocery item games to include maybe one item that's on "heavy rotation" that regular viewers know the price of, but vary the other items enough that it's not a dead giveaway.