Are you talking about principal rafters or common rafters. In the latter case you don't want some rafters touching posts because shrinkage of the plates will mean that the rafters on the posts finish up out-of-plane when the timbers dry.

If your frame is octagonal, your rafters are all hipped. In French carpentry you'd place a "gousset" and "Coyer" to receive the rafter. I was trying to find an example image but they don't really cover octagons so you'll have to make do with a description. The gousset goes diagonally from plate to plate to reinforce the angle. The Coyer goes from the gousset to the plate and provides a surface for the foot of the rafter to sit on.

In an octagonal structure I see even less of an advantage in removing the plate. Your rafters are going to all meet in the center so you can't raise the post and rafter at the same time. Better to have the posts all raised and solidly assembled to on another to provide a base on which to place the rafters.

Caveat: I'm not an engineer, and living in France, the codes that I know are the European ones.