How frequently -- as in after how many rounds -- should I dismantle and clean the gas tube?  I usually shoot between 90-180 rounds per range visit. In the past I've either broken down the foregrip (handguard, whatever it's called) and scrubbed the gas tube, or just removed the piston, a good rinse with non-chlorinated brakekleen followed by an blast of compressed air and a swab of Ballistol.

Just wondering what others do.

edit: heres Hickok vid
http://www.youtube.com/watch?v=Komm6tIUjtI