I find the most critical place to save a baseline version is after I've finalized the coastal outline. Also, if I'm working on a satellite-view style map, then immediately after finalizing the heightmap (a grayscale image in which the highest points are bright and the lowest points are dark).
I also tend to start a new file after I've finished each major terrain type and right before I start labeling. Although, with Photoshop's layering capabilities, I save fewer versions than I do with other programs, simply because I can simply turn off the layers of experiments that didn't work out.
Always save a new version right before major merges or other destructive operations, of course.