I have a 2012 5,1 and from what I researched, the Mac Pro has a 3 channel memory controller and was most efficient with just 3 banks (6 banks for dual processor) of RAM populated.
Mine came with 3 banks of 4GB modules on each CPU and I initially added a fourth for each, but I eventually exchanged them for 8GB modules, 3 for each CPU.
If someone has different information, I would like to know as well....
-howard
I guess I only answered your direct question regarding optimal memory configuration ... but I agree with the rest of the posters below that in reality you probably won't notice the difference when using the computer.