Exercise is good, good, good. Think of it this way, it increases the health and optimal functioning of so many systems in the body, including the endocrine system. Increased blood flow, means increased tissue health, and also increased exposure of everything to whatever estrogenic compounds you are taking. I have been doing P90X for some months now, and really pushing myself (for the past two months been very diligent about exercising 4 to 6 days/week) even in the intense resistance workouts. As Tony Horton says, "you have to bring it!"
I am in the best aerobic condition I have been in for many years. I am stronger than I have been for many years. I am feeling the symptoms of hip arthritis less than I have for quite some time. I have increased muscle tone and definition, and now the girls have a solid foundation to rest on--more developed pectoral muscles. Mine are not pointing out to the sides as happens with many males and I think they stand out even more now!
If you are concerned about increased testosterone following exercise (which is necessary for building muscle, even in women), take something that blocks conversion of T to DHT. That way you get the muscle building effects of T but avoid the masculinizing effects of DHT. In theory, this would actually make more T available to be aromatized to estrogen (the natural pathway ALL women make estrogen in their bodies anyway.)
Exercise is also known to increase growth hormone secretion by the pituitary, which can only help as well.
Finally, if you do some research on legitimate gynecomastia forums, you will find that exercise does NOT decrease true gynecomastia (as versus gynecomastia that results purely from being overweight which of course would decrease as one looses weight.)
Keep exercising--the healthier you are the better your body can respond to PM or whatever regimen you are on.
Good luck!