Hello, William!
I am a firm believer in "Till Death do we part," so I consider the death of one spouse to be the end of the marriage (I recognize divorce and annulment, too!). I don't always fill in the Marriage End Date for the profiles I manage, though.
I don't believe WikiTree currently has a set guideline for the Marriage End Date issue. The choice to use or not use that field, even regarding divorce/annulment, is currently left to the profile manager.