No, it's not necessary to refrigerate brownies before cutting. However, it is necessary to allow them to completely cool and come to room temperature before slicing. I know it's hard when there's a tray of freshly baked gooey brownies staring at you. But if you cut into your brownies too soon, you'll end up with a gooey mess rather than a perfectly fudgy and firm brownie that is able to hold it's shape. The cooling process could take anywhere from about 30 minutes up to 2 hours. If you're in a hurry, you can always stick the tray of brownies in the fridge to speed up the cooling process. However, I think you get the best texture and flavour when you allow the brownies to cool completely at room temperature instead of in the fridge.