I've been doing some poking about, and I think one could easily get away with 8Gb on a rMBP provided you weren't using virtual machines. I always have one sitting in the background using 3Gb, and sometimes have another with another 1Gb. With most things I use daily I'm pretty much on 8-9Gb used most of the time. Video work puts that up to 12Gb.
If you're not doing heavy video, and you're not using virtual machines, I'd suspect 8Gb would be plenty enough.
Remember too, that swapping out on an SSD is nothing like swapping on a HDD...