yes it's safe, if you do it right. There used to be a machine tire shops would have that would grind them down. It's not a new practice, it's a long forgotten practice. As long as you don't grind down to the fabric of the sidewall, you're alright.
it takes more than lightly sanding. And it's not JUST thin whites. Normally, RWL works too. Some better than others. Most guys use an electric grinder to remove the material. Just don't want to gouge the sidewall or remove more than required to expose the white underneath.