Scott - I think that the varnish is just a protective layer, especially for negatives that come into contact with paper in printing frames. Sometimes, plates were left unvarnished ("in the bright"), but I suppose they were ambrotypes, adequately protected by mat, cover glass and preserver.
Robert - that is undoubtedly correct. But I am acutely aware - maybe paranoid - about the uniqueness of each shot, and I (over-)compensate for it for taking up to three different exposures, hence I get a range of images that include ambro, ambro/neg and neg. I just hate the fact that I might have only one image and, being the butterfingers I am, damage it. I should get it right first time of course, but then again if I could do that I wouldn't be dabbling in photography - I would have done what dear old dad wanted, got it right, and would now be languishing on a beach somewhere...!