I'm going to be making a 3-tier wedding cake. Haven't made one in a while and am wondering what you experienced wedding cake bakers think look nicest stacked together:  12", 9" and 6" OR 12", 10" and 8"?  It will be a pretty simple design in white SMBC with ribbons and some flowers. 

Either would look good.  It really depends on how many servings you are wanting? 6",9",12" is 88 servings and 8",10",12" is 105 servings.

Yes, it depends on how many servings one needs more than looks.   Personally I like the more 'old fahioned' look of 12,9,6 :) 

Thank you!