I recently spoke with a woman in Toronto who had written a book and was looking to publish it. She was caught up in the old Catch22 that the fiction editor and the writer face together--how much should be changed to make the manuscript commercial enough for sale? I've heard stories about publishers wanting major rewrites ( 10,000 words or better) and people getting caught up in that machine.
So here's a little advice. Keep an open mind and look at all the suggestions an editor makes but make the final decisions yourself. Remember the best fiction editor will always be looking to make your book the best it can be and that should mean listening to your vision as well as having an eye to commercial value.