Best way to edit mail template

Hi,

What is the best way to edit a mail template?

We want to add an agent to a mail template and I just want to know the 
best way to do it. 

Should you create a copy of the original template or just edit the 
original?
If you have a copy do you use the copy or the original?
After you add the agent should you sign the template with your server ID?
Do you edit the inheritance settings of the template so that users upgrade 
the design from the modified template?
Do you modify the inheritance settings of the original template so users 
don't upgrade design from it?

Anything else I missed?

Thanks

Nathan Simpson
Australian Wool Handlers
Lara VIC  3212