I think another problem you'll run into is the volatility of prices.
I don't have first-hand knowledge, but I imagine that the automation of POS systems makes it very easy for retailers to change their prices on literally a moment's notice. There are a couple of good "find local gas prices" websites that probably have the technology/model you want, but I also recently read a story about a guy who noticed that the price of gas changed between the time he pulled into the station and the time he started pumping.
I'm not too concerned about this problem. I think if any major retailer starts to play these types of games, they'll get into trouble with the law pretty quickly. I'm confident in legislation against such bait-and-switch type tactics.
I have never had problems with gas stations switching prices on me. In fact, if they are raising the price, they usually have a time delay between when they change the big sign and the price on the pumps. That way, you may find that the the price on the pump is lower than the price on the big sign, but never the other way around.