I have always drained my cafetiere when pouring my coffee. From the last drops, you get the attractive golden foam that, on an espresso, is the crema.
However, no cafetiere plunger is so efficient that it traps all the coffee grounds. The free grounds are in the last drops of coffee you pour, and, settling in the bottom of your cup/mug, steep for too long, and impart bitterness. The last quarter of your drink may be unpleasant.
My recommendation is to allow the coffee to settle for a short while after plunging, pour it carefully, and leave the last 50ml or so in the cafetiere.
Making coffee, I follow Victoria Moore’s technique of using a lot (I allow 2tbsps for 300ml water) but plunging almost immediately. If allowed to steep for the standard recommended time of three to four minutes, coffee is more likely to be bitter, in my experience.