Here's a few bits from various web sites:
Proper fit is the most important factor to consider whether you prefer underwire or softcup styling. Underwire styles currently account for about one-half of all our nursing bra sales. With underwire maternity/nursing bras, it is especially important to obtain a proper fit, so the wire doesn't put any pressure on the breast tissue. Wearing an underwire bra while sleeping is not recommended. To prevent mastitis or clogged milk ducts, proper fit is critical regardless of whether youre wearing a softcup or underwire nursing bra.
Nursing bras should fit comfortably. Bras that are too tight can leave you vulnerable to plugged ducts and breast infections in the parts of the breast where straps or underwires block the flow of milk.
Avoid underwires, especially in the early postpartum weeks. If you do choose an underwire bra, be very particular about the fit. The breast's milk-producing tissue extends all the way back to your rib cage and up into your armpit. An underwire may obstruct the milk ducts in this area--besides poking and annoying you. (Underwire bras can be miserable to wear during pregnancy. The wires dig into your upward-expanding abdomen whenever you sit down.)
Avoid an underwire bra style while pregnant and breastfeeding. Since your breasts and rib cage typically increase in size and fullness, a wire doesn't expand and often becomes constrictive. Constriction of your breast tissue and rib cage can lead to an unhealthy fluid stasis, possible infection and overall discomfort. With the many aches and pains of pregnancy, don't let your bra be one of them!