You should always run all SQL Server services with the lowest possible privileges allowed in case the account is compromised. SQL Server setup makes the whole process of granting privileges a whole lot easier because it automatically creates groups with all the necessary permissions for you!
Figure: SQL Server now creates groups for all the SQL Server services with the bare minimum permissions for you
If you are running any SQL Server Service in a user account that has administrator privileges a user that compromises the account could do anything that administrator could do - including playing around with the registry with procedures like xp_regdeletevalue. So, if you use an Administrator account, you're in effect giving away the keys to the house. Is this something you want to do?