For a 3-tiered wedding cake are you supposed to use pan sizes that are 2 or 4 inches apart between the tiers? For example, I was going to do a 12x2, 10x2, and 8x2 but most of my books have 14x2, 10x2, 6x2. Is 2 inches enough room between each tier? A 6 inch cake for the top tier seems too small. Just wondering what most of you do for wedding cakes.
Thank You!

it all depends on the look you're going for, any additional decorations you need to place around each cake, and how many servings you need.

There is no right or wrong way. Although people do have preferences. Some prefer a 2" difference and others prefer a 3"or4" difference for looks. But in the end it really comes down to servings and design.

I would look at some of the pictures on CC and see what look you prefer. A 2 inch difference only gives 1 inch around each tier to decorate vs. 2 inches. For some cakes it looks fine. To me it looks more like a tower. For me personally I prefer the look of a 4 inch difference. 6 inch actually looks fine as the top tier of a wedding cake in my opinion anyway.

A 6" top tier is the industry standard. It's what commercial toppers are sized for and what a florist will expect if making a floral topper. My fav is to use 3" differences. 6/9/12 looks nice!

These comments and suggestions are great. Thanks guys!

If you are doing a 6,9,12 what would you do for the bottom if you were doing 4 tier. What do you think are the sizes of this cake? 

http://pinterest.com/pin/152066924891062956/

I do a 15".  I've done a bunch of them.  Serves 185  Great size for many weddings.

IMH the 3-4 inch difference looks better, the 2 inch between tiers give a tower effect, but some designs such as the floating rose work better with less separation.

It really is a personal thing as to which looks better.  I definately go for the 4" difference - 14, 10, 6 etc.

You can always stack your pans upside down to get an impression of the scale.

6,9,12,15 serves 185 people?

185, 188, close enough.