I have a 42" on the Viper without a sway bar. I took the sway bar off last year, and wish I would have done it sooner! Works well on or off the trail, allows the ski's to act independently and is more stable in the rough stuff.
With Simmon's ski's I didn't notice any more tippyness. With the Yamaha mountain ski it is a little more tippy on the trails, because they are so aggressive. If you have a 38" ski stance, it will handle better with a sway bar.
Nice thing about Yamaha's, you can take one of the "dog bones" out (to disconnect the sway bar), and take er for a spin. If you like how it handles, take the bar out, to save a few pounds. If not, its easy to put the dog bone back in!