I think that, in general, images that are in grayscale should be left as they are, and colored images should be adjusted for a neutral background color such as white or black. As one can see from your examples, adjusting the background requires adjusting the colors of the rest of the image. The first image has a cyan tone to it, and removing that tone reduces the blue in all of the colors throughout the image. I believe this looks better than the original, but I suppose there will be circumstances where it doesn't. As long as the adjustment looks good overall, I think it should be used, 99% of the time.
petopeto
paper color
This leads to the question: to leave the paper color, or remove it? It's a judgement call. In this case, I left it. It's a distinct tint that gives the work a particular, and obviously intentional, tone, and it doesn't look bad at all.
This one's a harder call:
The paper is slightly cyan; the second adjusts the whitepoint to match the page. In this case I'm leaning slightly towards the adjusted version.
Trying to reproduce the book accurately is a nice goal, but the difference in medium does need to be considered--a monitor isn't paper, and it's not always good to pretend it is. Textured paper is similar--it's an effect that just doesn't look good on a monitor.
(yeah, I'll fix the image tab formatting at some point...)