Never. There is no guaranteed way to breed perfect 6IV Pokemon. However, your best chance is to breed two 6IV Pokemon, one holding a Destiny Knot. This gives you a 1/31, or about a 3.2% chance of getting a 6IV Pokemon.
EDIT: If you don't have any 6IV Pokemon, your best bet is to breed two 5IV Pokemon, which gives you 3 guaranteed IVs. You'll get a 6IV one eventually, and when you do, you can use it in battle or for breeding, your choice.
I do strongly suggest getting a 6IV Ditto, though. Many people with Poeersaves are willing to make them for you, and it's a great help when breeding.
