On top of what PJL said, if you end it early, you will miss the alst minute of the show.
Content providers adjust their times because they know that people won't switch over until their current show ends.
And then maybe won't go to another channel as that show has already started.
TBS was famous for having their shows at 5 minutes after the hour.
DVRs have all but eliminated that concern.
Especially if you can get one with 4-6 tuners.