From what I had been reading, masturbation will have virtually no effect. You will not lose Estrogen, unfortunately nor will it reduce T levels. Also as Kelly said, I have read that many doctors suggest that to have orgasms post-op, it is good to keep the pre-op orgasms happening too. Yes, use it or lose it.
The only drawback I might envision is what is going through your mind as you practice? Would you be seeing yourself as male when you are female. That could cause some dysphoria.
Hugs and Happy, Happy, Joy, Joy!