This is not an issue for homeschoolers, but you do have to name a grade at some point. If you look for info,each school district has their own dates for K registration, so you can see if you can find the dates for local districts.
Spring Grove's date that I know of as of 2012, was Sept 1 and the child had to be five by that date. I know York City is different. Actually it is only a established parameter for schools and as a home schooler, YOU can decide when to begin K or other education.
The law in PA is age 8 for reporting, and so most kids are in grade 3 at that age, BUT, I have seen all sorts of variation. I know kids that are in fourth grade for their first evaluation. I know kids that were in second grade. I know parents that did not start "counting school (either k or 1 ) until after age 8 and for their first age 8 evaluation, the child was in first grade,
so it is really up to you.
Most parents like to keep the "grade" of their homeschoolers sort of in line with the majority, mostly for outside and church activities, as many Sunday School classes for kids are arranged by grade. So, I am sure this does not answer your specific question, but you can call her grade K at any time that YOU think is appropriate. Some parents start their kids early and others like to wait and have an older grade K student.
The Sept dates do seem to cause the most confusion, so your question is valid.
http://www.greatschools.org/students/academic-skills/150-kindergarten-ready-or-not.gs is a link that explains this confusion; Your child may be deemed ready in one state or type of school but not in another. In most states, a child must reach the age of 5 for public school, but the birth date can range from June 1 to December 31.
PA school accept students from age 4 to 6 for K and it varies by district. http://ecs.force.com/mbdata/mbquestU?SID=a0i70000004J3cq&rep=Kq02&Q=Q3195