www is often added as a CNAME but it will work as A also.

If you don't setup redirections to non-www to www. OR www. to non-www. So in that case there is possibility that search engine indexed both non-www. and www. URL and it's create big mashup.

Read below article by matt cutts

SEO advice: url canonicalization

I hope this will help you

For Nignx:

#run this command#
sudo nano /etc/nginx/site-available/example.com

#copy and paste below content#
server {
         listen 80;
         server_name 123.456.789.10 example.com www.example.com    ;
         return 301 http://www.example.com$request_uri;
}

Using this you can redirect your example.com to www.example.com

Adding to @hackosphere answer, the site failed to load when I tried duplicating the server name and returning the 301 to itself. Separating them to two directives does the trick.

server {
         listen 80;
         server_name 123.456.789.10 example.com ;
         return 301 http://www.example.com$request_uri;
}

server {
         listen 80;
         server_name www.example.com ;
}