I've noticed that a lot of wedding cakes on here have the third tier as a dummy tier. Do couples not freeze the top tier anymore for their anniversary or is that a trend that has fallen to the side?

I've never really heard of freezing the top tier until Cake Central. I guess not everyone does it, not in my culture.

Here's a history on freezing the top tier:

There is hardly a bride today who can't resist saving the top layer of her multi-tiered cake. Most couples freeze the cake with the intention of sharing it on their first wedding anniversary. The tradition has its roots in the late 19th century when grand cakes were baked for christenings. It was assumed that the christening would occur soon after the wedding ceremony, so the two ceremonies were often linked, as were the cakes. With wedding cakes becoming more and more fancy and elaborate, the christening cake quickly took a back seat to the wedding cake. When three-tiered cakes became popular, the top tier was often left over. A subsequent christening provided a perfect opportunity to finish the cake. Couples could then logically rationalize the need for three tiers --- the bottom tier for the reception, the middle tier for distributing and the top for the christening. As the time between the weddings and the christenings widened, the two events became disassociated, and the reason for saving the top tier changed. Regardless of the underlying reason, when the couple finally does eat the top tier, it serves as a very pleasant reminder of what was their very special day.

I'd say 99.9% of my brides dont' save the top tier anymore. They opt to come back in a year and get their free, fresh anniversary tier. But the top tier is as real as the rest of the cake ... it gets cut and served right along with the rest of the cake.