
Muddydogs is right - wait till she's midway between seasons. This is when her hormones will be at their most "placid" and the blood supply to the uterus is at its lowest, so there should be the least risk of operative and post-operative complications.
At 9 months she will probably be about to come into season any time, so keep an eye on her! And there should be little difference between a medium sized bitch like a golden retriever and a tiny bitch like a Yorkie, seasons-wise. Some are very clean, and others not.