I get so worried about these things! My DD, age 7, 1st grade, has missed so many spelling words just because of backwards letters, like b and d and p and q! However, I am forcing myself not to worry too much b/c she switched schools between K and 1st and this new school pushes writing and letters more than her old school did. She will get her numbers backwards every now and again too. So for us, I won't consider it a problem until late into 2nd grade, maybe as early as 3rd grade.
I also work with her at home here, having her write me stories about things that start with those letters. She absolutely loves to write! So I will ask her to write me a story about puppies. Funny thing is, she does fine at home, so I am pretty sure it's the stress of spelling tests and math tests that are getting her.
I would not worry until another year or so! Even though I know how hard it is not to worry!!!