In stalling WP Multisite with or without "www" prefix

I am not clear on the implication of installing WP multi-site with or without "www" prefix. On a single site installed this can be changed easily after the installing from the general settings. However this is not he case for the mulit-site installs in my understanding. In some of the hosts (like Site5 it seems that forces the install without prefix if I chose the install in my specific questions are:

1. Is it mandatory not to have "www" prefix for multi-sites install in subdomains?
2. What are the benefits/disadvantages to have/not to have "www" prefix in the primary URL
2. If a multi-site was installed in subdomains without "www" prefix can I change the primary URL to contain the prefix. If yes how can I do it?

Thanks in advance. Janos