I've seen instructions to roll out gumpaste with a little crisco on the mat and also some say to lightly dust with cornstarch instead. Which one is it?
Depends on what you are trying to accomplish. If you are using cutters, and your gum paste is soft, using crisco will make them stick to the mat and stretch then when you try to pick them up. Corn Starch will help them dry faster, keep their shape, and not stick so you won't lose your shape when you move them around.
BUT shortening creates a cleaner look without dust... It really just depends on what you are doing.
I always use crisco. Cornstarch seems to dry out the gumpaste faster.
I have zero luck with cornstarch. Crisco all the way. Not much. Not much at all....
Ditto to CakeInfatuation. I like to grease my hands with shortening when handling it, but if it starts to feel tacky on my work surface or to cutters, cornstarch is my friend.