Regardless of whether or not composers notated ritardandos at the end, usually the finishing of the performance of the piece should be with gradual slowing down. Of course, a few of the compositions might end up abruptly but the general feeling is like driving a car - you don't ever want to make a sudden stop because it would feel like a car crash, an accident. So you slow down. The same is in music.
The question is how much to slow down and how soon to begin this process?
Some people leave the ritardando to the chance and hope to achieve it naturally which sometimes works. But sometimes doesn't. Sometimes people start slowing down too early which makes the ending appear too predictable and boring, sometimes - too late, which robs the piece of solemn and proper conclusion.
Is there a system you can use to achieve an optimum ritardando? I think there might be. Here's my idea:
When you approach the end of the piece or an episode with a full stop, start slowing down at the final cadence. In order for your ritardando to be quite gradual, start counting out the beats of the measure out loud a few measures before that: "One, two three, four" (if that episode is in 4/4/ meter).
When you come to the place of the final cadence, start subdividing the beats saying to yourself: "One and two and three and four and". Feel how each subdivision becomes gradually slower.
It's very important to keep counting and slowing down when you reach the last chord. This will ensure that this chord is neither too long nor too short.
Feel free to experiment with these ideas in your practice and eventually you will find that some ways work better than others.
Diatonic Sequence in C Major: IV-V-I