Should a beard be trimmed wet or dry?

Dry, for the same reason as a haircut — wet hair lies flatter and cuts shorter than the setting suggests.

A comb sets its length against the skin, and anything that changes how the hair lies changes the result. Wet beard hair is straighter and longer under the comb, and it springs back shorter once dry.

That is most noticeable on a curly or coarse beard, where the difference between wet and dry length is largest.

Wet trimming also clogs a blade faster, which is why a tool that seemed to be cutting cleanly starts pulling.

Several trimmers on this site are rated for wet use, which is about cleaning them under a tap afterwards rather than about trimming wet. Braun states 100% waterproof on six of its eight products here — the BT5525, the BT9545, the AIO7565 and both body groomers, plus the 3040s shaver. The two exceptions say something weaker: the AIO3545 states washable attachments, and the EN21 nose trimmer states fully washable. Washable is a statement about the part you take off; waterproof is a statement about the body you do not.