My rules: 0. Always pay attention to the articulation - don't slur when you shouldn't and don't tongue when marked slurred. 1. Keep the air moving regardless of style (legato, staccato, in between) 2. For notes that are not marked slurred, tongue them deliberately - the harshness of the tonguing depends on the overall style of the piece and what the conductor has in mind 3. For slurred notes, I use the "du" or "da" syllable - soft attack with the tongue hitting the roof of the mouth as opposed to just behind the top teeth. The harshness would be controlled by what part of the tongue hits the roof of the mouth.