One of the first things discussed when beginning an SEO project is URL canonicalization. You may argue that the impact of having multiple URLs resolving to the same page may not have a huge effect on SEO (if you use it consistently internally), but it certainly doesn't hurt to normalize the URL so that only one URL serves a single webpage. Implementation is not too difficult for most webmasters, and there is a piece of evidence that URL canonicalization will have a positive effect on your SEO effort. You may use one form (www or non-www) of Url for all your internal linking, but you cannot guarantee that others will do the same on 3rd-party websites.