I know when i started yoga pointing them towards feet made the inside of my wrist have lots of pressure which at that point didnt feel good. Also then I searched in books and all books i came across had them backwards and not forward.
However i just tried it and now there is not that same tension on my wrists now when doing it possible because im more flexible now. I guess whatever feels good is ok but i prefer holding them backwards because im used to it. I think however if a stiff person gets tension in the inside of the wrists by doing it, it should be avoided to avoid possible injury further on. And i guess some of your students are stiffer than normal.
In the end i think whatever feels good for the practitioner is up to the practitioner to judge. but holding them backwards is definitely not wrong. Let them do backwards if that feels better for them.