Would you recommend re-excision in a breast cancer patient who received neo-adjuvant chemotherapy with no residual tumor on pathology, but sterile mucin at the edges was present?  

In the absence of viable invasive tumor cells at the tumor margin or in the absence of DCIS < 2mm from the margin, I wouldn't recommend re-excision. I would accept the excellent result of a pathologic complete response and that the presumed needed adjuvant radiation would be an excellent approach...