Really good reasons to use a center dowel include...
- If a client is picking a cake up and you're not delivering it.
- If there's a strange structure that would benefit from having the extra support.
- If there are tall separators between tiers that are larger than the separators (overhang).
That's all I can think of.
Don't bother with a center dowel if:
- The cake is two or three tiers and it's cold when it's delivered
- You're delivering two large stacked tiers and assembling the rest on-site.
Since most of my deliveries fall into those two categories, I don't bother with a center dowel. some people use one for every tiered cake, but if you do, you need to be aware of the following:
A center dowel is useless unless you build it correctly.
Hammering a sharpened dowel through a tiered cake won't do a lot to stop sliding or shifting if the cake is room temperature (i.e. soft), or if the dowel isn't anchored securely into the board below the cake.
If you put a dowel through the cake and not through the board the whole thing can slide right off the board if you hit the brakes too hard. The top tiers can also shift, and they'll drag the dowel with them. You'll end up with a dowel at an angle that's going through a bunch of shifted tiers.
The point of doing a center dowel is that it will keep the cake upright in case of rough handling. But to do that, it has to be anchored securely into the board. that means that you can either pound it through the board, or that it needs to be attached to the board with brackets or something like that. That would involve a trip to the hardware store, and would be a center pole situation more than just a center dowel.
Just be aware that a center dowel isn't foolproof.
If you drop a cake a dowel might keep it together, but it can crumble anyway. It might not even keep the cake together if it's not cold. And unless it's anchored correctly it won't do squat either.
In my experience the best way to make sure a cake will arrive in one piece is to make sure that everything is kept in the fridge before delivery so that it's cold when it's transported. Or to deliver things in sections that you assemble on-site so that there are no worries at all about tiers sliding off of each other.