Rebuild Index in system database I use SQL Server and have multiple indexes in system database whit index fragmentation 30% or more. 
I don't know that SQL server manage system database index automatically or I want to manage them. and manage index manually by I is a good thing or not.

thanks in advance
The SQL Server will maintain these indexes automatically.  If they have become fragmented you'll need to defragment them manually or via a scheduled job just like the user databases.