I agree that you should err on the side of a heavier tongue weight.  There's a significant limit to this, though.  The worst thing this can do is lighten the front wheels!  To the point that your tongue weight stops depressing your rear springs, and starts acting as a lever raising the front wheels, tongue weight is good.   This is why equalizing bars can help prevent some events from triggering sway.

IMHO, there are different causes of sway, not just one.  For example, the wheelbase of your tow vehicle, the length of the trailer, # of trailer axles, proper alignment of the trailer axle (probably a bigger factor than realized), proper and even tire pressures on the trailer axles and tow vehicle axles, cross wind section (center of pressure) of the tow vehicle and trailer, etc.  I think there are treatises on the topic available all over the 'net.

I've done what seems to me to be a lot of towing and never had sway control.  (Hmmm...  I've lost trailer brakes a couple of times, had blowouts, frozen bearings, debris damage, panic stops, ice patches, blinding rain, gale crosswinds -- the usual.)  There are some situations I'd definitely either (a) not let myself or any of my friends get in the tow vehicle if it wasn't staying parked, or (b) go out and buy a sway control device.  It's good insurance.

Oh, and in some states, if a trooper sees your rig sway, you can be cited for a moving violation.