I received this email from a piercer:

I written you before on the topic of aftercare products, and the use of pierced ear antiseptic and am proud to say the studio I work in now carries Satin! Anyway the purpose of this email is to address piercing lactating nipples. Over the weekend I had a very nice young lady request to have her nipples pierced, as we began the "paperwork" she informed me she was no longer breastfeeding but was still lactating. I have never encountered this before, so I phoned some of my  fellow piercers with a few more years under their belts, and asked some information. As it turns out this young lady was already at one of our studios on the other side of town and was told no. The other piercers and I all agreed, because we do not fully know what complications could arise that we would not do it. I told the young lady this, and explained why I wasn't comfortable preforming the procedure. I also explained that there could be a potential for mastitis, and suggested she speak with her doctor, or other piercing friendly medical staff. She promptly left in search of another piercer that would do the piercing for her. With all your years of experience, have you ever encountered this before? I've had little to no luck finding information on the Internet and wondered if you could shed some light on this. Thank you for your time, Myke The Piercing Lounge, Madison, WI www.piercingmadison.com

I told Myke that he was correct in declining to pierce someone who is actively lactating and directed him to this information from my book, with info on this point at the very bottom:

Nipple Piercing and Breastfeeding Many pierced women express concerns about breastfeeding, but there does not appear to be any evidence that nipple piercings negatively affect the ability to breastfeed. A normal female nipple has a multiplicity of up to twenty porelike milk ducts, rather than a single spout. Therefore, a nipple piercing of ordinary size and uneventful healing won't block them all. The ability to nurse could be impaired if a troubled nipple piercing causes excess scarring. When jewelry is removed from a well-healed nipple piercing, some colostrum or milk might seep or flow from the empty channel. Leaving out your nipple jewelry during breastfeeding is safest for your infant, although some women do successfully nurse with it in place. Jewelry removal eliminates the most serious risk of your baby choking on a ball, ring, or bar that becomes unfastened. You also diminish the potential for other nursing problems such as difficulty latching on or damage to the soft tissue of your infant's mouth. Another concern is that body jewelry might be a source of bacteria that could enter a baby's system. Some women have had success nursing with flexible jewelry like PTFE barbells instead of metal. Do not wear a retainer that has an O-ring closure because it is not sufficiently secure to stay on during nursing. If you decide to take out your jewelry and leave it out until you are done nursing, the piercing may shrink or close up by the time your baby is weaned. If your piercing is fully healed, there is some chance the hole could remain open. It may be possible to encourage a well-established channel to stay viable by passing a small, clean insertion taper through it on a regular basis. If the piercing has sealed shut and you wish to be repierced, it is best to wait at least three months after you stop nursing to allow the tissue to normalize.