I think you can use both of them as you like. It's depends on you or the situation. I usually use 私. I can see わたし in stories for children or in poems or in posters on walls or in my friends' mails and letters. I feel 漢字(= polite or casual, smart), ひらがな(= casual, friendly)
I don't like to study.
= 私は勉強するのが好きではありません。or 私、勉強が好きじゃない。
Sorry, I wish I could tell you well. (^_^;)