I agree with Michael et al. When in doubt, throw it out.

However, being of mostly Scottish heritage (with a smiggin of Native American), I tend to be conservative. I usually save the stop bath and the two-bath fixer for paper development. For film, I save nothing.

When I an faced with doing a multiple enlargement assignment, like recently, I cover the paper trays with Saran wrap between sessions. This helps preserve the chemicals and prevents accidents like dropping dried prints or other "foreign material" into the trays. Developer will not last more than a day's operations the way I perform - s l o w l y....