My son did the same thing and he learned to write in Kindergarten because I was afraid that he would learn the wrong way if I tried to teach him. I only taught him to write his name.

He never made errors in reading the letters and when he accidentally wrote a letter backwards, he could see that it was wrong immediately because a b where a d was supposed to be would cause the word to be wrong. But if he accidentally wrote a 5 backwards and didn't catch it right away, especially when he did long division or something with a lot of numbers where he also had to worry about keeping columns straight, it would sometimes cause him to make a mathematical error. This was something he had to learn to check. He had to make sure his 2 was really a 2 and not a 5 that he accidentally wrote backwards.

He did Handwriting Without Tears but I would catch him going back to his old way of forming letters when he thought I wasn't looking. I notice that he even makes checkmarks backwards from the way I do it. It just seems totally wrong to me the way he does it but it looks okay, so I let it go.

I think my son's problem was more of a motor learning thing. I think he needed more writing practice on certain letters and numbers than most kids, just like he needed more practice to learn difficult dance routines. His auditory memory and visual memory (especially for words) is great but his mild motor learning problem is something he has to deal with.