Usually the Release is useful if you want to build some automatic sustain into a voice - eg. strings, which are a little "too fast" for a slow tempo song and tend to cut off too soon.
For that you'd use positive values. Using negative values I've found up to now doesn't seem to do too much sonically. Going negative would help to cut off a voice more abruptly if it's needed but in general most of them cut off cleanly enough at close to zero. I can't see what benefits there are in going to -20's - apart from turning off the sustain pedal

John