MkVII is "proper". But I think MK7 is easier and looks better. The MK Five guys use "V" because it's easy, I think that's also a factor in why "6" caught on as much as it did. Certainly the websites being named with the number itself also plays some roll in it. That's why you see more "MK6" on golfmk6.com and more "MKVI" on Vortex, because the section's name.
Personally, I like MK7 and am sticking with that for now.