It is better to form the letters top down because ultimately the writing will be faster and more legible, because it's easier and more accurate to pull a pen across the page than to push it. Teachers used to insist on this but I think they don't care any more, or they care but not enough to do anything about it.
There is a real difficulty in teaching this to small children. For small children, it can be difficult to put enough pressure on a pencil or pen to result in a visible line when pulling from top to bottom: their little hands just aren't strong enough. Some end up pushing instead because pushing puts more force into the paper, which can make the line more visible for the same reason it makes it slower and more likely to poke holes in the paper.
All three of my children initially had this problem. I didn't correct this in the eldest, my daughter, because she was at the time behind on reading and writing and we felt those were more important than proper stroke direction. She was (and is) also double jointed and used (and uses) a different grip because the normal pencil grip was painful for her. In retrospect, I might have done things differently, as now, a couple years later, her letter formation is still sloppy and barely legible. She did eventually learn the normal stroke directions in cursive but rarely uses cursive, and her stroke directions remain wrong when she prints.
My elder son is two years younger, and I insisted on correct stroke direction from the beginning, which for him was age 5. He could handle it better because his hands are bigger for his age - they have always been as big as his sister's even though he is 20 months younger, and thus were larger when he started than hers were when she started - and he is not double jointed. His letters are highly legible and his printing is reasonably fast.
My younger son is another two years younger, currently age 4, and I've been trying to insist on correct stroke direction for him as well. His hands are not as big for his age, and he started at an earlier age; although he is not double jointed, he has difficulty making a visible line with a pen or pencil without pushing, so stroke direction has been a battle. I've switched him to washable markers, with which a visible line is much easier to make. He is now willing to start at the top when he remembers or is reminded; it takes a lot of reminding, but I think it will eventually be possible to retrain him to use the correct stroke direction, unlike his sister where it seems to ingrained.
Bottom line: fix it if you catch it early enough, and don't be afraid to give the kid washable markets to make the normal stroke direction easier to use with small hands.