While I have only done wedding cakes for friends and family members (not yet licensed!), I do hold "tastings" for them. Since I use doctored cake mixes and buy my fillings from a supply store, tastings are usually pretty easy for me.
I bake mini loaf cakes, and usually split a mix into two or three portions, and flavor each of them differently. My flavors include white cake: vanilla, lemon, orange, and almond flavors; chocolate cake: vanilla, almond, and orange flavors; spice cake with and without nuts; carrot cake with and without raisins and pineapple; and marble cakes in chocolate/vanilla and chocolate/coconut.
I have 8 or 9 flavors of filling that I keep on hand (I really use them often enough to justify having them on hand!).
I slice the mini loaves into 6 or 8 slices and serve the fillings in individual custard cups. Everything is labeled, plates, forks, spoons, and napkins (usually in colors to match their wedding colors) are supplied.
I have everything ready and on the table a few minutes before "my guests" arrive. Before the actual tasting, we sit in the living room and discuss their plans for their cake, go over pictures, colros, sketches, etc. until we all know the "final" plan. Then, it's off to the tasting table! So far, everyone has just loved mixing and matching the different cake flavors, fillings, and icing to find their perfect match. Some couples have very simple tastes, or know exactly what they want. Others think they know what they want until they are given the opportunity to experiment. All have had a wonderful time and are very appreciative of the effort I have made to make this tasting a very fun and memorable part of their wedding planning.