Andrew Whitechapel posts an exhaustive blog on when to use ReleaseComObject in Office add-ins.

Bottom line for me:

  • “If you’re sharing an AppDomain with someone else you should probably never RCO, because other components in the AppDomain that you don’t control won’t be able to accommodate your RCO.“

And remember, if you aren't using the shim technology to load your add-in into it's own appdomain--if your add-in is loading into default domain, inevitably you will be sharing an AppDomain with someone else.