Of course varnish protects both ambros and negs. But originally, ambros were matted and cased. Ray Morgenweck sums it up neatly:
"Sulfur compounds in the air eventually tarnish and darken the silver. The surface highlight areas buff down to a sheen. Gradually the whole plate looks greyer. Tintypes develop rust blooms.
If you seal the plate from the air, by using a cover glass and a mat spacer, the image will last essentially unchanged for years. Some of your best plates can be the bright and white ones, prior to varnish. So, theres no reason not to seal them and keep them that way. An 'underexposed' ambrotype on black glass, left white and bright and sealed up, has a wide tonal range and can be stunning under good lighting. I use bulldog clamps to hold it together, and Scotch 3M photographic tape (black, available through Pearl Art) to seal the edges.
Id say, if you dont want to varnish, get a good method of sealing. If you do, dont bother with other varnishes, learn to use the sandarac well."