What type of stainless steel are you talking about, the shiny silver type?
Ok, let's get serious about this. If you are dealing austenitic stainless, slow cooling may work against you if the temperature range is within the temperature range where you can promote sensitization. If you are working with martensitic stainless, slow cooling may work to your benefit. If you are working with ferritic stainless, slow cooling may cause you to linger in the temperature range were you can experience embrittlement.
One would have to know the alloy you are working with and the temperature of the metal when you start to control the cooling rate. If the stainless is already cooled below 875 degrees F you may be fine with slow cooling. If it is above 1300 degrees F it may work against you. again, the alloy type comes into play.
Insulated blanket over austenitic stainless steels would harm stainless steels more than doing good. 304, 316 grades are never hardenable by any heat treatment. Only allowed heat treatment is Solution Anneal: Heating and Soaking @ 1000-1040 Deg C followed by water quenching.
Slow cooling would promote Cr-Carbide precipitation(sensitization)resulting in poor corrosion resistance and should be avoided.
Thanks.