Predefined Administrative Accounts in Oracle 11gR2

At my company, we run a query/report to determining which accounts are expired or will be expiring.  There are two predefined Admin accounts I was curious about.  They're called MGMT_VIEW and XS$NULL.  I don't think we use these accounts for anything, I think they're just there when we setup a new instance of oracle on one of our production Linux servers.  My question is, should we keep them up and running and make sure their passwords get changed routinely and don't get locked out or can they just be expired and locked (incase we need them later for something).  Any help or info on this matter would be much appreciated.

Thanks,

Kyle