How do you know the level of additives used at the stations that are not Top Tier? Of course they do need to be at the EPA minimum level, but beyond that you don't know if they are not the same as the Top Tier stations, do you? That's kind of the point of Top Tier that it is an outside agency that is guaranteeing a superior level or additives, but that doesn't mean a station without Top Tier certification does not use additional additives.
Bottom line is that they may not spend the extra "3 cents" but you don't know that they don't.
You're right, you don't know for sure if unbranded station x is using additive above the minimum required.
But you have a pretty darn good idea...because otherwise they would either be top tier or promoting the fact that they were using more than minimum.
And there have been testimonials from gas truck delivery drivers describing the process and that there are differences.
I think the consensus on this forum would be if you used unbranded fuel for the life of your vehicle vs top tier the resulting carbon deposits on pistons, valves, etc would be statistically different.
There is certainly marketing data to that affect. Its certainly your right to believe and do whatever you want.