Do surgical margins matter for benign breast conditions (after excisional biopsy surgery)? would re-excision be necessary if no malignancy is found?

No: If benign, margins are irrelevant. Hope this helps.

Possibly yes: Carcinoma insitu, is a pre cancerous lesion, it has the potential of becoming cancer and is treated aggressively. Getting a clear margin around it is a good idea and recommended as it is with malignant lesions. Carcinoma insitu is not benign but it has not grown into a full cancer. Just making sure this is not what you were referring to.

Yes for some: Dcis needs to be completely removed and the wider the margin the better up to 1 cm. Positive margins should b reexcised. Certain other benign breast lesions likely don't need complete removal always. Negative margin for dcis means 2mm or more.