Weight loss will show in different areas for every person. 10-15lbs may not have much change in your breasts. The best advice would be to have a consultation with an experienced board certified plastic surgeon to discuss your past weight loss experience and formulate a plan for you.
While it is always a good idea to be at a stable weight before surgery, usually 10 to 15 pounds won't make a noticeable difference, however in very small women it may, if you have lost 10 or 15 pounds in the past and saw a noticeable difference in your breasts, I would suggest waiting until you reach your goal weight.
It is always ideal to be at a stable goal weight before surgery. Losing 10-15lbs post surgery can effect the breasts very little, or tremendously depending on where your weight loss occurs. Some women notice very little change in their breasts with weight fluctuation, while others see very large changes. I recommend being as close as you can to your ideal weight before the surgery if possible. I hope this helps.
Pregnancy and weight change are the two main things that can affect your surgical result. Breasts are composed partly of fatty tissue, so if you lose 10% of your body weight, your breasts might get smaller. This may or may not affect your level of satisfaction from your result. So for example, if you weigh 100 pounds or less, your 10 to 15 pound weight change might affect your result. Hope this helps. For more information on this and similar topics, I recommend a plastic surgery Q&A book like "The Scoop On Breasts: A Plastic Surgeon Busts the Myths."