All of the above.
IT IS GOOD to install a relay triggered by your original "ignition run" circuit, especially if you have other switched circuits on that wire The relay, however, has nothing to do with "MSD or not."
The switched wire is a LOW CURRENT "trigger wire" just like a relay.
Hook the thing up to either "ignition run" or to your "ignition run relay" and forget about it.
Only reason to run an ignition wire through a separate toggle is something like anti theft