The layers of a bottom tier of a 3 tier stacked cake (14-10-6) have only baked 1" tall. The layers of the other tiers are average height. Would you bake a third layer for that bottom tier, or would you just build it up with frosting?

Definitely bake another layer (or layers)... it should be at least 4" tall or slightly taller than the 10" and 6". Didn't you fill the pan 2/3 full with batter or more than 1" to begin with?

On my tiered cakes 12" or larger I like to use 1/2" foamcore board same size as the cake, then I ice them as one- cake-n-board, and place on display board. The added height on these larger bottom tiers create a nice balance, i think.

I just did the Cakeman Raven Red Velvet, which allegedly uses 3-9" pans for 1 recipe. So I just used a 14". Baked 2 of them on 2 separate nights, and they are exactly the same height, and perfectly flat, actually, not domed at all. So I'll go ahead and do a 3rd one tonight.