As mentioned above it depends who is taking the photos and for what purpose. Most of the photos we dig up and post around here originally come from auction archives. In that case I'm glad they don't manipulate the image any more than they already do. Consistency is the key. If you are selling two posters and the paper has darkened on one, the photos better reflect that -- and hopefully be consistent across the whole site.
To further correct color, I use X-rite Colorchecker Passport.
http://xritephoto.com/colorchecker-passport-photo
An example from my thread:
http://www.allposterforum.com/index.php/topic,11559.200.html
Photograph the Colorchecker under lighting conditions used for your photo, then import the file into Lightroom or Photoshop to create a preset for all photos taken under the same conditions. White balance and clipping (full grayscale) can also be adjusted. A calibrated monitor is also recommended.
Yes, this is the only way to get an accurate, consistent photo -- include a color card in every photo and adjust exposure and color temp in post to the same levels every time.
Mel, your use of any sort of "auto-adjust" setting will give different results for
every single photo.
The software can't possibly know the "correct" brightness level, so it instead bases it's calculations off the image itself, same for color temp. Even if you took 2 photos from the same auction house that were shot at the same time and post-processed in exactly the same way -- if one was a white background poster and one was a full bleed dark background poster, the "auto-adjust" algorithms would treat them much, much differently.
At that point you might be more pleased with the look of the posters for online use, but it probably no longer accurately represent the physical item. Based on your clean-up jobs of the Library of Congress posters, I don't image that is your intent anyway, though, so it shouldn't be an issue. Just be aware that others responding to your question might be coming at it from a very different place.