J.
I agree - there's no law that a child can't start K when they are 6. On the other hand, since she is a girl she will already have a maturity jump on the boys. You might check with the district and see if they can give you an idea of how closely parents stick to the guidelines. The age range in my son's k and 1st grade classes has been pretty big (he has a July birthday, so like your daughter will be, is one of the youngest.)
On the other hand, our relatives in Michigan have a Dec. 1 cutoff, so lots of 4yos starting in K - I think it's really about what your child is ready for and what you feel good about.