I started wearing underwire after DD was around 6 months and haven't stopped since then (so through feeding a toddler, another pregnancy and then newborn). I've never had a blocked duct even when I get engorged (and happens quite severely when DD1 spends a night at her dads as she misses a very large bedtime feed). I also wear underwire bras to bed more often than not as I enjoy the comfort and security of underwire. It seems to have worked fine for me so I guess it depends and you'll never really know unless you try. My breasts are a 10D-DD whilst breastfeeding if that means anything and I always make sure to be fitted when purchasing bras to ensure a proper fit which I'd assume might help?