I'll second Marcus on bucking wherever possible, and my reasoning is two-fold:
1. I don't care what anyone says, Olympics have uneven points of force around the circumference of the seal, thus meaning that if the copious amount of Vulkem you have shoved in the hole and coated on the rivet ever fail, there are 3 uneven points which are more prone to formation of gaps due to minute oscillations/vibrations of the trailer and thus the aluminum skin on which the rivet head sits. Whether that is 1.00001 times or 100 times more likely to create a gap than a flush head rivet, it is still more likely to create a gap. Period.
2. Olympics in bulk cost me $0.230/rivet while the buck rivets I use cost me between $0.018/rivet and $0.030/rivet, or an average of $0.024/rivet. For all intensive purposes, 10% of the cost of Olympics. When replacing all the existing "exploding head rivets" on a PO repair, re-installing the skins to the door frame, constructing a new vent, and patch installations (all the cases where I had access to both sides), I probably used 250 rivets. I just saved ~$50. If I felt I had been jeopardizing the leak proofing of the trailer, that would not have been worth it. Instead, it was a bonus that paid for my freshwater pump.
Granted, I like Andy's disclaimer that he always states: This is all assuming correct installation of the Olympics, and or proper bucking technique. If either are installed poorly, they WILL leak.