Print Developer:
Yeah, cover them and see it the print still comes up in a reasonable amount of time compared to fresh. I find open trays of Dektol go off after about 5 hours or so, and so can define a single printing session.

Film Developer:
You only get one chance with film. I wouldn't leave more active film developers out (of unstoppered bottles) at all. I use all film developers one–shot, anyway.