G.W.
I found a couple of articles that may help you:
http://www.todaysparent.com/schoolage/article.jsp?content...
http://life.familyeducation.com/cognitive-development/ear...
I'm no expert, but I'd say she's fine. I used to do it all the time, for fun, to kind of test myself. I'd write whole pages upside down and mirror image just because I could. If you do a search, you will see that many people see that as a "developmental problem" that she will grow out of, and many consider the ability a gift.