The contribution to print permanence achieved through selenium toning is directly proportional to how complete the toning is. Most of us tone just enough to get a slight change in image tone. This also means a slight bit of protection added. There are still lots of silver molecules in the print not bound with selenium, and therefore, still "vulnerable."
A print toned to completion is usually too red/brown for most tastes. Some papers will go completely fire-engine red if toned to completion. Such a print would be well-protected, but not very desirable. Other papers exhibit little change in the toner, which means that no toning, and hence, no protection, is happening.
It is worth noting that the entire brouhaha about selenium toning and permanence comes from a study of microfilm, which was toned to completion in selenium. This, indeed, protected the film. Extrapolating these findings to prints resulted in a lot of speculative misinformation about how even a bit of selenium toning would provide "archival protection." More recent research has debunked that to a large extent; the degree of protection is dependent on the degree of toning.
So, go ahead and tone, but do it for aesthetic reasons, not to provide protections for your prints. If you want your prints to last, use two-bath fixing, don't exceed fixer capacity, use a wash aid, and wash your prints well.
A couple notes on selenium toning. Firstly, when used properly, it is not very "nasty" at all. If you buy liquid concentrate, e.g., Kodak Rapid Selenium Toner, and mix your working solution from that, risks of inhaling selenium powder are minimal.
Use print tongs or nitrile gloves to avoid skin contact, but don't freak out if you get some on your hands or even have to dip your hands in the toner working solution to fish out a print stuck to the bottom from time to time. Just wash your hands immediately after contact. Remember that Ansel Adams and his whole generation of photographers manipulated prints in toner with their bare hands; they had a lot of exposure and we don't hear of many cases of selenium-related diseases among those photographers. Yes, we know better now, so minimize contact.
Clean up spills and wash trays and vessels well and you should have no problems. If you're sloppy, you'll see where you've missed cleaning up, since selenium toner dries to red blotches on surfaces, which can be cleaned up later if you catch them soon enough. Selenium will stain trays and sinks; cleaning up can prevent the stains, but the stains themselves after cleaning are non-toxic. Prints toned in selenium are not toxic either.
If you feel you need to limit your exposure to selenium, avoiding Brazil nuts would be your first choice. Not mixing toner concentrate from scratch would be your second (I don't, I just buy the liquid concentrate). Not toning prints in selenium is way down the list.
My issue with the way some use selenium toner is that it is environmentally unsound and wasteful. Selenium toner working solutions can be filter and replenished and reused almost indefinitely. I've written enough about that here and over at the LF forum that a simple search on my name and "selenium" should turn up lots of relevant information should you be interested. Short version: use your toner until toning times get too long, then add a bit of concentrate to bring the activity back to where you like. Filter the toner with coffee filters or lab filter paper before and after each use. An advantage of this method is that the unpleasant ammonia odor disappears almost entirely with a seasoned toning solution.
Best,
Doremus