Setting the domain preference should be the first thing that should be done by ans SEO guy. With a single domain fixed, i.e www version or no-www version helps to get SEO benefits in the future.
Reasons Why You Should Set Your Preferred Domain:
1. The Panda Update
Panda is responsible for penalizing the sites with duplicate content. For Google, the www version and non-www version for the same sites are different. Google on seeing the same content, penalizes the site and then there is loss.
It is advisable to the webmaster to choose 1 version and make it as fixed from the Google Webmaster Tools.
2. Double the Power of your Backlinks
When preferred domain is not fixed, then link juice gets dispersed. When the link juice is dispersed, our website which was supposed to have 2*N number of backlinks, ends up having less number of backlinks.
It is crucial to choose one domain name version and use it for link building.
Google does try to canonicalize matching domains, but they don’t guarantee it, as Matt Cutts’ explains below.
3. Personal Preference
There is no such hard and fast rule. Choose the version with which you are most comfortable. No matter what version you choose, stick to that version.
Also visit SEO Karo for latest info.