You know where I'm having trouble with this whole DNA thing is this - I have done so much pedigree research on so many dogs asking the questions digging through the rumors that when I see a dog that is 'clear' where I KNOW there is a lot of documented fanconi - I still couldn't bring myself to risk it.
For example when you look at Hip displasia - if EVERY puppy in 2 litters was x-rayed you would be better off to breed to the Fair puppy in the litter of Good/excellent than to breed to the Good puppy in the litter of poor or fail. Has to do with how that is passed on.
I think - even with the DNA for Fanconi - until we have some longitudinal data I would still err on the side of caution so would not consider breeding to a clear if I know there was a lot of fanconi behind him or her. That's just me - but I couldn't - at least in theory.