I just installed MySQL on Debian 6.
With PHPMyAdmin I noticed there are already 4 MySQL users:
debian-sys-maint (localhost) root (127.0.0.1) root (localhost) root (myhostingcompanyurl)
Are all of those necessary?
The answer is "sort of". The debian-sys-maint one is definitely necessary, Debian uses it to perform automatic maintenance like rotating logs.
The three different root accounts are because MySQL treats your host address as part of your account, so even with the same username, you might get a different account depending on how you connect.
If you don't want to have different permissions for people calling themselves root depending on how they connect to the server, you could create a