I have read to let the fondant sit on the cake for 12 hours before stacking. Is this neccessary?
I have a cake with perishable filling, I don't want to refrigerate the fondant overnight. I was hoping to cover it very early in the morning the day of the wedding.
Any suggestions?

I have always stacked my fondant cakes right after covering them and have never had a problem. You should be fine.

I stack mine right after covering. I do put a layer of confectioners sugar down first - just in case I need to move it a bit - otherwise it sticks like glue. Just brush away any extra and make sure to pound a wooden shish-ka-bob stick down through all layers and foam-core plates.

I always stack as soon as I cover them. The longer that you wait, the fondant dries and can crack.
Edna

Thanks for the advice!