The material you are firing
The material you are firing upon can make a difference. Firing on Thinfire or sprinkled powder is less likely to leave sharp points.
But, you may be firing things that are less than 6mm thick. In this case the glass is retreating from the original footprint due to surface tension trying to be about 6mm thick. Things to think about:
make pieces thicker
fire to a LOWER temperature, so there is not so much chance for the glass to draw in.