I noticed that even the best fitting bras need a couple of wears to feel perfectly comfortable. The first time I´m wearing a new bra means I feel the bra on me all day. Often the band will feel slightly too tight, and the underwires feel stiff. After a couple of wears I don´t feel the bra at all, it´s like a second skin.
I´m guessing that the fabric, elastic and underwires bend & stretch to adapt to my body. Probably this happens not just with bras --- jeans, sweaters and socks all mold themselves to our curves and angles. Also, the first wash removes whatever the fabric has been treated with, softening it a little.
This doesn´t mean that an uncomfortable bra will get comfortable with time. If the bra feel uncomfortable the minute I try it on, I don´t take any chances and so I don´t buy it. But if the bra sits perfectly, feels right, and is comfortable; then I know that the slight discomfort that will develop after a few hours of wearing it will pass away.
One exception is the underwires digging into the sternum. When this happens I bend the tips away (on a table top), or even sew a small dart.
How long does it take for a bra to adapt to your body ? Do you also bend the underwires on tables ? What is your bra-comfort problem zone ?