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.
Use wooden or plastic dowels or plastic pegs. Cut down so that when pushed into the cake they sit completely level with the cake surface. The next cake tier can then be placed on top supported by the dowels. Use 4 pegs or dowels per cake, 5 if the cake is very large.
Place pillars in required positions on cake and push a wooden or plastic dowel through each pillar and then through the cake until it hits the board. Rest a ruler across 2 pillars at a time and, with a pencil, mark the dowel precisely where the pillar ends. Now move the ruler to another 2 pillars and repeat so that each dowel has been marked (4 pillars - move the ruler 4 times, 3 pillars - move the ruler 3 times).
Carefully pull the dowels out and re-mark them all to the highest pencil mark. On occasions, the cake surface may not always be completely level and taking the highest mark means that when the cakes are erected, the pillars will not sink into the icing at all. Now cut the dowels on the highest mark with a hacksaw, clean and carefully push back into the cake in the same holes - place pillars on.
Remember: as long as the dowels are all cut precisely the same height to the highest pencil mark the cakes cannot sink into each other.