Hi Dan,
To me, "gain staging" is being aware of, and controlling the signal level at all points really.
Recording level, channel strip and insert input, input and output of all plug-ins and processors. Including on bussed channels and the main bus.
It is not always critical, but at the same time- it is just good practice to be consistent, which IMO saves a lot of time and improves the end result.
In your description, the specific project with many channels may become cumulatively too high summing into the main bus. Therefore it might be handy to place a trim plug-in in at the first main bus insert to set the level where desired if for example you are running some plug-ins on the main bus.
I always have a metering suite in the last main insert, and since Mixcrafts main fader is post insert, I always leave it at unity (0) and set my mix-down level via a fader in that last insert.
The specific target may depend on whether I am "mastering" within the project, or mixing down and mastering the song independently, as a set or EP for example.
Everything is variable, thats just a general outline and starting point.
Someone else might say thats all wrong, but thats what I do. Hope it helps answer your question.