The idea is that you dont have to pull up with the upwards angled guns. You just fly directly under and hold the trigger.
I dont see night or day having any difference in the tactic used with those guns (whose name I can never remember, and when I do, never spell), but I believe that aircraft armed in such a way gave up on other weapons due to weight/balance concerns. So, upward guns for pure buff killing (or strafing ground targets, if you are attitude chalanged like yours truly), or forward guns for fighter killing and attack?