i dont know about a 6" lift, but on my 3" lift i have front and rear sway bars. i cant really noticed it unhooked putting around town, but taking high speed corners(my truck can still do that w/ 33's) and zipping around in general there is alot of body roll without them. i tried driving down from my local mountain one time without hooking them back up... couldnt go thru a corner at more than 20mph, were before it was more like 40. plus the feeling like the rigs gonna roll right over.
all depends on how stiff ure spring are, how much cargo u have, and most important, your driving style. i'd agree with everyone else... try it without first before u go buying brackets and stuff to make it line up.