Not only ok but recommended by Doctors. Regular breast stimulation has been proven to prevent breast cancer and if you do it often enough can make them grow.
Breast growth is controlled by hormones and the hormones are released naturally in response to nipple stimulation. Prolactin (milk) and oxytocin (love) are released and if prolonged enough and regular enough, say 10 minutes, 3 times a day or more, the glands in your breasts will start to grow in preparation to make milk. Most women get about a 2 cup size increase. It is simulating having a baby suckle except that you wonรขโฌโขt make any milk unless someone actually does suckle. Maintaining high prolactin levels is what makes it work. It could take weeks before you notice the difference and it wonรขโฌโขt work if you skip a few days but it does work and itรขโฌโขs free, and it also prevents breast cancer.
http://www.ehow.com/how_4482979_risk-breast-cancer-nipple-stimulation.html