If you are doing a 3 tiered wedding cake, the bottom layer being a 10 inch round, would you use a 12 or 14 inch stand? Thanks

I'd use a 14", but there's no wrong answer. Depends on the cake design, flowers and whether or not the cake will also be on a cake drum?

After icing, the cake will be less than 11" in diam. but that's too small to put on a 12" stand IMHO. If you need to add flowers at the base, for example, that's going to be unsupported too. Add a 12" drum and you'll really need that 14" base.