Maintaining System Tables?

I have a question that just popped into my head,

Do system tables maintain themselves or is it my responsibility to make sure system databases and tables are maintained/defragmented/etc.?

I currently run maintenance plans for example to rebuild and reorganize my user databases, but i was wondering if it is necessary to include system databases into those plans as well.

Thanks in advance for your response.

Reymar

You can't touch system tables.

Ok, I was just wondering...i figured so but just wanted to be sure i wasn't losing out on any optimization.