Good Morning, Katja,
With high-quality enlarging lenses, good prints are usually possible at almost any available aperture; nevertheless, the generally-accepted wisdom is that it is helpful to stop down two or three clicks to reach the optimum performance. With most black & white papers, using the lens wide open will usually mean very short exposure times; for convenience when dodging and burning, exposure times of ten to twenty seconds are often preferable. The use of f11 or f16 is, therefore, a typical practice for small (up to 8 x 10) prints.
Konical