Dowels are a must for supporting tiered cakes covered in sugarpaste (the roll out icing) whether stacking the cakes straight on top of each other 'American style' or using pillars or separators.
For Stacked Cakes - Carefully Insert one dowel into iced cake and mark where it sits flush with cake surface. Cut dowels to depth so that when pushed into the cake they sit completely level. The next cake tier can then be placed on top supported by the dowels. Use 4 dowels for small cakes and 6 dowels for larger cakes. For Pillars - Sit pillars onto cake surface in their required positions and push a plastic dowel through the centre of pillar and then through the cake until it hits the board. Use a pencil and mark the dowel precisely where the top of pillar ends. Carefully remove dowel and use pencil mark as a guide where to cut all dowels.