When I raced 1/4 midgets back in 2001 we siped an ss22 right rear for day time racing to dissipate heat... Worked okay for the time and place my dad did it we came from dirt modified racing... Since I been racing karts in 2005 I haven't seen it done yet, the thickness of rubber changes dissipation, that's why guys have tires cut.
You don't have enough horsepower in the stock classes to be pulling those edges of rubber along the track (which in turn pick up the race track like mini-grooves) and throwing rooster tails. It also tears the rubber from the tire which isn't desirable.
Needling a tire will get you the benefits of building, dissipating, and maintaining heat in a tire without the sipe edges cutting into the track and tearing the rubber. You can needle a tire in much less time than siping as well.
Now, if you are running grooved tires on a big HP open, and you don't mind replacing tires often, siping can be used. I'd still prefer to needle the size tires we run on karts (grooved or slick.)