I think it is a personal decision really but does help with support, especially if you tend to get *full* overnight. I usually was so tired I would cradle Emad next to me and wake up with them all hanging out anyhow...lol
Until recently I produced a lot of milk during the night, so personally I was more comfortable in a night time bra--some support but no underwires and very comfy. I purchased them at Motherhood Maternity and they were affordable.