I did this because of errors otherwise. Some are not documented on Technet...
1. Primary Site Server (local SQL): Local Administrators = Systems in the Primary site (e.g. remote DPs, AISP, etc) *** Some roles on remote systems give errors if you don't do this.
2. Primary Site systems (e.g. remote DPs, etc): Local Administrators = Primary Site Server
3. Secondary Site Server: Local Administrators = Primary Site Server
4. Secondary Site systems (e.g. remote DPs): Local Administrators = Secondary Site Server + Primary Site Server *** I forget why I had to add the PS computer...
Maybe someone else can explain better...