basically, I like to diffuse when I print because I can control the amount of diffusion. I usually try about 25% of the exposure time. I often use an acetate negative sleeve and move it around under the enlarging lens. The result is that it flattens the neg by about .5 of a contrast grade and lightens the print by about 10 or 20%. I use this technique with teens who have slightly rough complexions. It helps a lot. I like to keep the effect subtle. Sometimes on the camera lens I will use a #1 Pro Mist Black. However I don't like a "romantic" misty look--it always looks corny to me. I'd rather start with a sharp contrasty negative and make adjustments when I print.
There a a zillion ways to diffuse. Years ago, I worked in a portrait studio. We shot split 5x7 negatives and had an retouch machine that covered imperfections with pencil lead. But now, I only shoot 35mm.