You won’t have a penelty for not using a 301 redirect on the non www version of your website. If that were the case thousands of websites would be knoced out of google. You site will be indexed twice, BUT google will pick a version and treat the other version as sandboxed.
However, many SEO guys use a 301 redirect to ensure that every link is pointing to the same place and any PR or value is redirected to their main domain.