Just curious...in those wedding styles where one tier is like 6" or more tall...is that served as one slice or is it divided halfway up with a board, and served as two tiers?
I serve it as one slice, but instead of 1 inch by 2 inch slices, I make them a little more narrow. It's nice to have the 2 layers of filling showing. Just make sure the bride knows that they may need to adjust the size of their plates for a longer piece of cake. I really like the look of the taller tiers.
Frankly, I think I would do it as (2) 3" high cakes and cut wider pieces. Basically since a 3" is 75% height of a 4" I would figure the servings out that way so for example if an 8" x4" round serves 24 (according to Wilton), then an 8"x3" would serve 18.
I don't think it's wrong to do it either way, but serving a 6" tall piece is a BEAR plus you'd have to charge way more per serving to make it fair (for you). The highest I would go without making it two sep. cakes would be 5" IMHO