I will only cast a vote against putting them on the rail. I KNOW it is popular, and lots of people do it, and lots of people have not had an issue with it. I did it that way on my old boat. They worked well, and were out of the way.
Fourteen days out of Hawaii headed to San Francisco in the middle of a North Pacific gale we were swept across the deck by waves several times. Green water falling across the deck hit the back of the stowed solar panel hard enough to break the stanchions it was attached to. I see some of the "cruising" boats out here that carry all manner of stuff all over the outside of the boat, and the damage I imagine they might have sustained in such conditions is frightening.
Granted, these were conditions that were pretty extreme, but not beyond what any boat crossing an ocean should be ready for. It gives me pause about using the rails for permanent storage of anything. We are all colored by our experiences, and this was the worst weather I ever hope to sail in. But it really gave me an appreciation for what moving water can do to things tied to a boat. It's why I keep clear decks when sailing.