I did some experiment not too long ago.
Made a bunch of identical mini-prints. They are like 3.5x5 inch in size. Then tried different toning on each. From what I understand, toning to completion means there are no more silver left that are unconverted. That would mean, with selenium, all silver is converted to silver selenide. With this understanding, I took my paper and toned it for 1 minutes, 2 minutes, 4 minutes, 8 minutes, and 16 minutes. Obviously, there are significant color change at 8 and 16 minutes mark.
I washed all these prints very VERY well.
I then took these and toned it in brown toner. With peculiar property of brown toner, (thinner it is, faster it tones), many of them shifted from selenium color to brown very fast in wash water. I recall even 8 minute one shifted somewhat. 16 minutes one were basically unchanged - meaning it was toned to completion in selenium leaving nothing for brown toner to convert.
You could do a similar experiment if you really want to find out.
In my own process, I tone for color first, and permanence second. Little toning is better than no toning in term of longevity so even with partial toning, I am contributing to image permanence. Some toner attack dense area first where as some toner attack highlight area first. You also have an option of using two toners and protect both.
I think we get caught up in archival thing a bit too much (myself included from time to time).