I always read that with two or more tier cakes you need to put dowels so the cake doesn't sink. But do you have to put the cake on top with a carboard to hold it up. It never really specifies. All the website's I look at show's it but as a guide to know where to put the dowels in. Any help would be grateful.
Yes - all tiers above the first need to have a cardboard support underneath (or a different material, such as foamcore). Without this, the dowels have nothing to hold up and will puncture the cake above.
Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:
The above thread contains illustrated links to common cake support systems with complete directions.