Some women do experience more plugged ducts after wearing an underwire bra, and others don't have a problem with them. It all depends on how you — and the bra — are made. (If you have widespread boobs that extend into your armpits, an underwire might not work out.) Anything that consistently presses on your breast tissue can block milk flow and lead to a plug. If you have been wearing an underwire without any trouble, there's no need to stop. However, if you've been prone to plugged ducts, your bra might just be the culprit.
If you feel the need for an underwire, look for ones that are designed for nursing (such as Medela underwire nursing bras). These bras are designed in such a way that the wire rests much farther back, past most of your milk ducts and usually doesn't cause problems.