Should I teach myself to play the cello or piano?

So I have played the violin for over seven years (since 4th grade) and really want to learn a new instrument, but can not afford lessons. I really want to start playing cello or piano. I have always heard that you should not teach yourself how to play without a teacher; but I was wondering if it would be advisable for me to teach myself (especially cello) since I already play the violin. What do you think?