I am with gsdsar, and to wait for her to mature before spaying her. However, if you are unsure of taking on the responsibility of an intact female, then by all means have her fixed. It is better than an unwanted pregnancy.
I spayed my female when she was 4 years old, and she was ready to get back on schedule by the second day, post surgery. When I purchased her, her breeder recommended to spay her after 18 months old. I had a GSD as a teenager, and she was spayed after her first heat w/o any complications either. She lived a healthy life, apart from poor hips in her old age.
However, every dog is different and to reiterate, it is a personal decision based on your lifestyle and responsibility to an intact dog.
Janka vom Grunenfeld, BH (d.o.b. 10/12/07)