yes it really makes no difference whethere you use it or not as long as you are consistent and do not have duplicate content on the www and the non www so make sure you redirect from the www to the non www or the other way around
Doesn't a 301 redirect only work if your on a Linus server? I was reading about this lately because I want people who go to website.com to automatically go to www.website.com only because I've heard of instances where the non-www site ranks differently then the one with the www. I did it in Webmaster tools, adding both version, verifying and then selecting the one I prefer ed. Is this enough?
Thats true.. But I will suggest you to prefer non www version over www version and set a 301 redirect from www version to non www version.
Selecting preferred domain is good but not enough, your site should have one unique URL i.e same content should not be accessed from 2 different url.
you can use any of them as ur domain name but strict to only one, suppose u r using www version then make redirection of non www version to www version