No, you don't have to use kanji, however, if you want, you can use kanji. We have kanji and using kanji is needed but there are cases that we can chose if we use them or not. For example, friend(s) is ともだち and kanji for it are 友達 but I prefer writing 友だち because it seems soft to me though it depends on the person, on her/his opinion. It also depends on the word.