There are several things that cannot be performed before Shekiah, and several things that cannot be done after. (For instance, one cannot daven Mincha after Shekiah.)
What if one is unsure if Shekiah has passed or not? Do we follow the general rules of Safek d'Oraisa L'Chumra and Safek d'Rabbanan l'Kula? Or do we say that we're always machmir? Or perhaps always meikel?
(I'm not asking specifically in regard to Shekiah. The question could be asked to any Zman; I just picked Shekiah as an example.)