I would also agree, if the owners really cared about her or at least were looking for her... they should have found you by now. If you want to get her fixed, then I would say after you've had her for 3wks-month, do it. I know where I live if you provide food and shelter for a "stray" and can prove you tried finding owners the dog is legally yours after 30 or 60 days of providing food, water, shelter. You should check out laws/bylaws of ownership and etc in your city.
Good luck and I'm glad it's going good!