Why is the final moment on a self-tape important?
You filmed your audition and it's pretty good. You sent it to casting and now they're watching your tape.
You dressed to suggest your character. You look like you could be that person in that situation. Check.
You have a moment before the dialogue starts—a reaction, a thought, or an activity. Check.
You did your research and the pace of your dialogue and the tone of your acting matches the tone of the show. Check.
You understand the "why" of the scene and are hitting all the right moments. Check.
Then, as soon as the last line has exited your mouth, you cut the tape.
Remember, your self-tape audition isn't just an audition to prove you can act. You are also proving that you are experienced and you know how to be on a set today. Casting directors are looking for that, and they want actors who understand the big picture.
A rule of thumb to keep in mind: If it wouldn't happen on a set, it shouldn't happen on your self-tape. When you're on a set, the film keeps rolling after that final line, to get that last thought, breath, activity, life. As soon as you've said your last line, they don't yell "CUT!" The director and the actors let that last moment breathe. (Even if they make a jump cut to the next scene in editing, that's still how they film the scene.)
So, even if you're new to the business, have just started sending in self-tapes, and have very few credits, you can put the casting director's mind at ease by showing them you understand how that final moment works. That's what the pros do. That's what you should do too.
Now, go out there and book it!