i dont know! i called her yuki for ages but when i read the vitabu it alisema yuuki. sooooooooo i guess it dosent realy matter. wewe just have to decide what wewe like better.
I think it's to differentiate between the kanji for "snow" and the kanji for "gentle princess", as in Yuuki's name. Both are pronounced identically but are written with different characters. In English translations, "snow" is written as Yuki while "gentle princess" is written as Yuuki.