It is hard to predict if the 7-10 lb weight loss will impact your result. It is your decision to move forward. Best of luck.
The breast reduction alone will help you lose some weight (depending on how small you are going maybe about 1 lb). Any weight you lose after this could potentially come out from your breasts, but given it doesn't sound like you are losing a great deal of weight, you are not likely to end up with "sagging" breasts. I have had a patient lose more than 50 lbs after breast reduction surgery with some deflation and a little bit of excess skin in her breasts, but if you are talking about losing 5-10 lbs this is less likely to happen.
A lot of it depends on where you lose most of your weight with weight loss. Some women don't lose weight in their breasts, only their abdomebn, flanks, hips. I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with and who is a board-certified Plastic Surgeon certified by the American Board of Plastic Surgery.
Harvard Educated, Beverly Hills & Miami Beach Trained, Double-Board Certified Plastic Surgeon