We tried to time it so that the last show of the night would start at closing time, so that we didn't have to run an additional show for folks who arrived after the last show started, but before the park closed.
That usually meant an extra-long break after the next-to-last show. A few times, the theater operator screwed that up, and forgot to place a hold on the show. In that case, we usually just let people arriving just before closing to go straight into the theater to see the show in progress, in the hopes that they wouldn't demand a full show after the park closed. (Once, a family did demand that - so we had to call park control to allow us to start a new show five minutes after park closed. They did, and the family saw their show - and the theater operator got reprimanded for screwing up the timing, since security had to stay late in the area to clear the guests.)
In practice, the final show of the night was almost never filled. Only once did I see a full show at Bear Band at closing, forcing us to run a second show after the park had closed. That was on the old cast member Christmas party night, back when (a) those existed and (b) Bear Band ran the Country Bear Jamboree Christmas show, which, naturally, all the Christmas party-goers wanted to see.
I bet Fantasmic has gotta be brutal to stop the line!