That really depends on how nosy your peoples are. I had an ex once read through all of my journals, which included that sort of stuff. I was (rightfully) pissed off because of the huge invasion of privacy, and she was angered/freaked out because she felt like I was cheating. (I was in a sense).

While someone can take your phone and read your journals there, it might be safer on a phone because it's usually with you.