Depends on the sentence. It does depend but on it's own would be shouldn't have.
Should have. The confusion arises when the conjunction "Should've" is used, which sounds very much like "Should of".
Should Declare War The timing of the United States entering the "Great War" (to be later called World War I) came at a time when our political leaders realized that our world is becoming smaller....
Apologize sincerely? More details please..
It would be nurses should.