One of the risks of breast augmentation surgery is capsular contracture. This happens when the scar tissue that forms around the implants either becomes or shrinks so that it compresses the implants, making them hard. The idea behind massaging your breasts is to keep the scar capsule larger than the implants so that the capsule cannot shrink. I am not aware of and study that has proven this effective. Besides, by putting the implants under the muscle, every time you move your arms, you technically would be massaging your implants.
Sadly, I know of surgeons who tell their patients to massage their implants and then blame them for their contractures saying that they did not do it enough.