Is it good practice to assign a system account for database owner? I can see the owners of my databases usign this query

SELECT  name ,
        SUSER_SNAME(owner_sid)
FROM    sys.databases


What are the things I should consider when choosing an account for database owner (account fro dbo user)? I'm thinking about a scenario when I assing normal admin account for database owner and when the time passes this account is disabled from AD when the ownwer of this account leaves his job. Is it good practice to assign a system account for database owner?
I advocate having databases owned by a Windows account (login).  

If using Mixed mode, sa could be used, but sa is a bit anonymous for my comfort.  Users owning databases (sometimes seen on dev database servers) doesn't work in production.

Like @nopol I prefer using Windows groups to reduce the number of logins under SQL Server, but groups can't own SQL Server databases.