Another reason to acknowledge an assignment at 1800 is that you're now released to the assignment. If not acknowledged then not released, so throughout the evening something worse could pop up they can change your assignment and call you at 0001 when your LSR starts. If you acknowledged the prior assignment they can't call or, at least, you don't have to answer, since you've been released.

OTOH, if a better assignment pops up, you can call them and ask to switch assignments. If they see an advantage to them, they'll do it.