I think that is pretty much an old wives tale. My understanding is the "R" rule originated before the days of ice and refrigeration, so naturally the "R" months were cooler in nature, so the oysters kept better. But many people still swear by it.
However, I DO think oysters harvested in colder water are better. They seem to be bigger, more salty, etc.
I certainly don't think it is unsafe to eat a raw oyster in the middle of the summer. (unless you have a compromised immune system, in which case you shouldn't eat them anyway, regardless of the month).