I haven't read the research on this, nor have I asked my baby's doctor his opinion. But I breastfeed no matter what my bg is. Because of breastfeeding, my bg has generally been really good, but I have found that occasionally, especially during the night, my bg will go really high for no reason that I can identify. For me, the benefits of breast milk outweigh the concern that it is sometimes too high in glucose. If I planned to breast feed until age 10 or something (!), weight gain might be a concern, but I don't, and so far my daughter is normal weight and very healthy.
(Not to mention the fact that research shows that children who were breastfed are less likely to become obese or have diseases like type 2 diabetes later in life).