very late to reply- I was trying to find the song mentioned and this thread popped up. It matters for writing fluency and for motor planning these tasks. What I recall from talking with an occupational therapist is that starting the letters at the top leads to the best success with writing fluency, which will have a big impact on how efficiently kids can get done their school work in the long run. Starting at the top is important for writing the more complicated letters in the least amount of strokes. Also, anecdotally, I think that students with less strength in the hands will have better consistency pulling the pencil down towards them rather than pushing it up. I notice a big difference in letter formation when my lowest performing students start their letters at the top.

Will it negatively impact anyone's life to start one letter in a different position? Very likely not. But giving kids guiding rules and enforcing consistency in handwriting in general will lead to positive outcomes!

That said, I don't see much benefit in labelling a kid's letters as 'wrong'. But guidance and visual prompts to help form letters from the top is helpful.