There are benefits to having more then 6GB of ram, but there are huge diminishing returns. I doubt you will need anymore then 6GB for Photo Editing, but that depends on what your doing. If you a pro working with very large images and lots of them then maybe its worth it. If you just want 12GB of ram for prestige then that's ok too.
I have built about 6-10 computers for work that run ASUS P6T V2 with 12 GB of memory. They all run great, have not had one issue. They are all running GSKILL 1333 with the speed manually set to 1333 in the bios.
These computers are for our 3D modelers who use 3DSMAX, Photoshop, and a number of game engines. They all love the computers, there is no lag in 3DSMAX after coming from a socket 775 quad core. Many have actually asked for the specs to build their own personal computers from.
I would guess there is no benefit to having more then 6GB memory if you are photoediting only.