I recently read that you shouldn't sit a baby up until they can support themselves up because it can mess (deform) their back. My daughter only ever wants to sit but she cant quite support herself yet. Whenever she is in a reclining position, she basically does a baby situp (throws her weight foward) to get in a sitting position. Is it safe to buy her a little chair that sits her up straight? Or should we wait until she doesnt need support to sit up?