The plastic dowels are fine if you're doing just 2 tiers, but if you're doing a 3 tier, I would definitely have a large wooden dowel going through the middle in addition to whatever other dowels you use.
I use plastic drinking straws cut to fit (not the bendy ends) and they are very strong... I place the same number of straws minus 1 as inches of the tier I am stacking....example...if I am placing an 8" cake I used 7 straws...one in the center and 6 under the cake evenly spaced.....6" cake use 5 straws underneath, etc.
I like either cardboards or plastic dowels on large tiered cakes for safety. SPS systems are good too but more expensive.
Wood is good but I don't trust the little tiny wood dowels that wilton sells. I rather use thicker ones. But more time consuming to cut. A dowel holding all the tiers to me is a must.