Letterpressing large areas of solid color is difficult; you often get uneven coverage, splotchy color and texture. I was printing a broadside design that used large areas of color, and my friend Christine recommended that I dampen my paper.
Despite doing letterpress for almost 3 years now, this was my first experience printing with damp paper (Crane Lettra 110#). It took extra time and effort to dampen the paper (mist each sheet, blot dry, store in plastic bag under a rigid board and a weight), but it was worth it. Note the difference in color and coverage between the blue (printed dry) and the yellow (printed damp). The blue came out very light compared to the ink color I had mixed, plus it was blotchy and had a dark line around the edge. The yellow, on the other hand, had even coverage, a better deboss (damp paper is softer than dry), and the color turned out bright and rich (damp paper takes more ink than dry).
When you dampen paper, it expands. If you are printing multiple colors and registration is tight, make sure your paper is the same dampness for printing each color, otherwise your colors will end up different sizes. This is more noticeable on large prints. Here, the yellow and blue were designed to be the same width, but because one was printed damp and the other dry, there is a 1/16" difference.
Also, when damp paper dries, it can warp. To flatten, lightly re-dampen your finished prints, and stack under a rigid board with a weight on it. I used a shelf from my bookcase, and the books from said shelf. Wait until dry.
Overall, printing damp is great for small runs of large, high-quality prints. But the extra time and work aren't worth it for large runs of smaller prints, or where imperfections in ink coverage are less noticeable.