just a data point. I booked a compact at IAH few weeks back. I am only "Emerald Club". My reservation said "Emerald Aisle Service". Did not think it was correct. At any rate, went to the counter. They said, I can choose any car from Emerald aisle if I wanted. I checked again with the folks by the car; as well as at the exit point. Everyone said "no problem picking the emerald aisle with this reservation".

I was not upcharged thankfully!^

Dont know if this is true for all stations.

But, if I reserved a compact, I will always check to see whether they will give me emerald aisle (I really do not deserve emerald aisle per the rules).