The advice normally is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break. It continues on to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.
This is true, but only on rounded tack and fused pieces.
When the process is a slump, there is not enough heat to round the edges. So, the edges will be sharp whether the break was on the heat up or the cool down.
How can you tell in a slump process when the break occurred?
If you can put the pieces of the slump back together and they fit perfectly, the break was on the cool down, as the piece was already fully formed.
If the pieces do not fit together perfectly, the break was on the heat up. This is because the break occurred, and then the two (or more) pieces slumped independently, thus leaving slightly different shapes at the break line.
There is a special case here, of course. Sometimes the break is only a split in the bottom, that does not come all the way to the top of the piece. This split (or splits) occur when the heat up is too fast. The top becomes plastic while the bottom is still brittle/stiff. The weight of the hotter, more pliable glass overcomes the strength of the cooler and heat stressed bottom, causing it to split. More information is given here: Diagnosis of Breaks
There is also more extensive information on diagnosis of breaks in this blog entry on slumping cracks.