Dear tidepool: If an abnormality has been discovered on mammogram, the type of biopsy recommended is usually based upon where the problem is. The folks who recommend the biopsy are aware of your anatomy and likely scheduled accordingly. One advantage of the prone table (lying on the table on your tummy) is that, for once, gravity is on you side.
Generally, part of the decision to recommend and schedule such a biopsy is making a judgement that the particular anatomy of the breast and the location of the "target" are amenable to the procedure. They know the size and anatomy of your breasts from the xrays; so I'd assume they've concluded it's possible, or they would have recommended a different option.
I agree with surgeon that they only do it if they think they can get to the suspicious area. However, sometimes I think they are too optomistic. They tried for 45 minutes putting me in some very uncomfortable positions (my arm through the hole along with my breast) trying to get the calcifications within their sampling site. They never did and I had to have an excisional biopsy.