What size (round) layers would I use for a wedding for 100 people? 12,10,8,6?

12,10,8,6 will come out to 130 using wilton's serving chart. If you want to get 100 servings exactly, you'll need a 12,9,6.

You might get a few different answers here, as there are different serving charts. First, you need to know if they are saving the top tier, and do they require 4 tiers? I use Earlene's serving chart, and a 12-10-8-6 would yield about 98 servings, that's if I include the top tier in the servings.
her servings are found here:

http://www.earlenescakes.com/ckserchart.htm

you can also find Wilton's servings here:

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

Earlene's is the one I looked at and 12, 10, 8, 6 came the closest.

So, forgive my ignorance, but is the difference between the two charts just a matter of how it is cut and/or the size of the slice? I don't see a size of slice for Earlene's.

Would 98 slices be pushing it for 100 people?

Should I have a bigger cake to account for 2 more slices or should I bank on 2 people not eating cake?