I too have used underwire bras, nursed all 4 of my kids, but would
have to say that I have had some trouble with them.  The worst offenders
for me were the nursing bras with underwires, when I would open them from
the top my breast was being still held or cupped underneath by the bra and
this pinched the tissue enough to cause congestion and inflamation, so I
went to a standard bra and slip it over the breast no pinched tissue.  The
other times I had trouble were sleeping with the bra on, the underwire put
too much pressure on the glans on the side , swelling & pain was under the
arm pit and a little forward,  and last but not least poor fitting bras in
general have caused me trouble in the past.  Sharon DEM, Tucson,Az.