In general, having pierced nipples does not affect breastfeeding unless something goes wrong in your healing (extreme scarring, mastitis, etc.). There are about twenty milk ducts that exit the nipple, and a single piercing should, in theory, do little to no damage to this system. That said, you should take the jewelry out while you're actually breastfeeding. If the jewelry is left in, it is possible for a bead to come loose and become swallowed or aspirated by the child.
However, there is a remote chance that an infection from a nipple piercing could damage milk ducts and jeopardize breastfeeding, even after the piercing is removed (see the mastitis entry as well). Certainly it's worth considering the risks. It's a remote chance, but when it comes to offering the best health for your child, many future parents will choose to draw the "better safe than sorry" line very far on the "safe" side.
You can always get them pierced afterwards!