Should I comb my beard before trimming it?

Yes, downward first so you can see its actual shape, and again between passes to lift the hairs that were laid flat by the previous stroke.

A beard grows in several directions, and combing is what makes those visible before the trimmer is switched on. It also reveals the real outline, which is rarely where it looks in the mirror.

Between passes, combing lifts the hairs the blade pushed down. Without it, the second pass appears to remove far more than the first — which it does, because those hairs are now standing.

Any comb works. A beard comb with two tooth spacings is easier on a dense beard, and a cutting comb with a handle is what clipper-over-comb work needs.

Combing wet is a different matter: wet beard hair sits straighter and longer, and trimming it produces a shorter result than the setting suggests.