Sure, both rear wheels spin (posi-traction), but there are a zillion other factors that make the car go left or right (Weight tranfer, physics, road angle, if front tires are slightly off center, etc...). My rear end usually pushes to the right, if memory serves.
Sure reasons to spin the tires from a roll:
1.) Cold road/weather (under 60 degrees)
2.) Cold tires
3.) Hard, old tires.
3.) Dumping the clutch
4.) Slamming the gas down in 1'st or even 2'nd if conditions aren't good.
5.) Power shifting (Keeping the gas planted while shifting) (Can break tires lose in 3'rd).
6.) OF COURSE, RAIN.
* This is for stock or near stock Gen II's. Another story for tuner cars (600+ HP). Even harder to control.
Learn how to modulate that clutch, and the car will take off like a rocket, even in sub-par conditions. If you feel the back end starting to swing, LET OFF THE GAS ASAP. The car will straighten up. Unless you are an experienced driver that can adjust with the steering wheel. Power slides anyone? Even experienced drivers need a lot of seat time in the Viper before pushing it. The Viper WILL bite you, if your not careful.
Like Ron said, power is fun - and dangerous.