I also only use fuel from Top Tier retailers for the improved cleaning levels. I also occasionally use an STP fuel injector cleaner as a preventative measure.
However, I am a little skeptical about Shell's marketing of the nitrogen enriched gasoline.
An article I stumbled across from 2009 (see below) talks about how they suspended funding all alternative fuel research and another source stated this as a reason why they began a more aggressive marketing campaign for their enriched fuels. This may or may not be true, as unfortunately I cannot find the source again.
I will still purchase from them if the need arises (ie, they are nearby and I need fuel), as they are on the Top Tier list.
Scientific American article