so far i havn't really had any probs with stacking but i have only done smaller 3 tier cakes. now i have a wedding cake to feed 400. i guessed it will need to be 6-7 tiers and i have NO clue how to make that much cake hold up! i have only used wooden dowels and some newer plastic dowels, but i don't think that will be sturdy enough for that large of a cake
You can use SPs system. Here is a link about SPS:
If you are doing this in one single stacked assembly, then please do everyone involved (you and the paying customer) a favor, and use SPS. You won't regret it!