This is one of my first wedding cakes and I am having trouble deciding on what size tiers to use. The bride said that we can include the top tier in serving. Any thoughts?

Is 186 the number of guests she invited or that have confirmed they are coming? a 16"/12"/8" would serve 180 based on wiltons chart. No everyone will have cake so that would probably be plenty.
Here is the link to the serving chart.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

186 is what she thinks will show and she doesn't want her bottom tier larger than 14. :/