Steel absorbs heat much faster than glass, so the glass suspended on the steel is cooler than the suspended perimeter during the heating and cooling cycles of the firing. This does not apply to slumping when the glass is supported on the edges, as so little of the glass is touching the mould at the start.
The fact that the steel “bleeds” the supported glass of heat while the unsupported parts heat up, requires slow heating with or without periodic soaks on the way up to ensure the glass and steel are the same temperature up to about 540C or the upper strain point of the glass.
I tend to be very cautious, and for 6mm pieces heat up approximately like the following:
100C/hr to 100C, soak 20
150C/hr to 200C, soak 20
200/hr to process temperature
When cooling, the steel is in closer contact with the glass, no special considerations are needed, so the normal annealing soak and cool are used.