Recently a friend asked me about "www" prefix to domain.
What to use: www.domain.com or domain.com?
You have to decide once because changing it after is a bad idea mostly because of search engines indexation.
Here are two great threads about that
1. WWW or not WWW, what to choose as primary site name?
2. What’s the point in having “www” in a URL?
I personally think that "www" is redundant in our days but it's important to understand technical reasons described in link 2 to make right decision. I knew about cookie domains but I didn't realised that it can be an issue as it not restricted anymore in "no-www" way.
If you can avoid "www" - go for it.
Remember to setup a silent redirect on web server for users who forgot to add "www." or did that for no reason.