Correct posture is important to comfortably producing a good tone on the horn, as with any wind instrument. Using a poor posture can result in wavering pitch, inadequate air support, pinched or hollow sound, and various other playing difficulties.
While playing, the player's back and neck must always be in line. It is recommended that hornists (and most wind instrumentalists) sit with their backs off the back of the chair and feet flat on the floor for a slight "leaning forward" feeling. The most important thing to remember is that the posture must be as upright as possible without causing any excess tension; the entire body must be as relaxed as possible to avoid tension creeping into the sound.
Only the tips of the fingers should be placed on the valve levers. This allows for faster depression of valves, allowing the player to change pitches properly.
Correct posture will allow one to adjust dynamics more correctly and produce a better sound.