I'm sure this question has been asked before but I can't locate it in the forum....
How many layers of cake high can I go before a cake board is needed?
The rule of thumb is you need a cake board and dowels every 4".
I often make dessert cakes (rather than celebration cakes or special occasion cakes) where I bake my cakes using the collar method so that they are more than 2" tall, which I then tort, and a filling, ending up sometimes with 5"-6" of cake/layers/frosting. I don't use cake board between the layers for these types of cakes.
But if you are making tiered cakes, definitely be sure to use those cake boards, otherwise you tempt fate and a tipped over cake!