Well you obviously want the game to use the destiny knot to pass through the 5 IVs other than Atk, because having those at 31 is more important than attack at 0. But, if by chance you do get a shiny that carried the attack over, you'd want the parents to have been the 31/0/31/31/31/31s, not the 6IV, because then you would carry over the 0 atk and not the 31 atk. Getting a 31/0/31/31/31/31 child will be 1/31 chance, because you'll have one IV randomized (with the others being exactly as you want them) and a 1/31 chance that the random IV will be what you want it to be.
If you used the 6 IV one, then you have to worry about the game picking the 31 Attack IV which would mean you could have 31 Attack AND another IV you don't want. Remember you're going until you get a shiny, which may mean you have to make a sacrifice. You'll want as many of the other IVs to be exactly as you want as possible.