Cedar is a fairly soft wood. Expansion and contraction will cause the post to work against the concrete. While roofing asphalt will isolate the post from direct contact, it will also seal the bottom of the post. Moisture will collect at the bottom and will not be able to escape.
I believe the correct way to handle this, is to wrap the post in black felt builder's paper, set the post plumb, tamp rock into the hole a few inches deep, then pour concrete. Cut the paper just above the concrete or the ground level after it sets. You can drive framing nails one inch into the post after you wrap it to add more anchoring power.
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