For me, nuke 6.6 not being avalible to the "masses" isn't a problem. In fact, even if it was avalible, I more than likely would not upgrade unless it contained an important fix.
Why? The last upgrade I did left half my modules not working, and removed all modafications I'd done to the core script, which were (if I may say myself) extencive...
Next upgrade I'll be making for my webpage will more than likely be 7.0...
However, if I was to start a new webpage based on nuke, 6.6 would be a good starting point