There is no cut and dried answer to this, besides it can be dangerous. So you must know what you are doing. All cases can be different for best sound. Generally it is wise to have your preamp grounded and let ground run to the rest of your gear through the IC-ground. But even this is no safe bet, because sometimes your amps inputs may be isolated from ground. Somtimes you do get better sound letting your entire system float. But sometimes you do not, besides, as Riley has rightly suggested, this can be hazardous. You'll have to experiment and listen and best measure your chassis to ground for stray currents. The less current the better sound generally.
