When I rough turn I make a tenon at the base of the green wood blanks
The problem is once the blanks is dry the tenon becomes oval. I have not
yet found an ideal way to true the tenon before placing it in the chuck for
finishing.

I normally do rough outs and leave a tenon as opposed to a dovetail recess. If
you leave the tenon larger than that which gives you "full coverage" with the
chuck jaws, you can jam chuck the dried piece with the oval temon, true up the
tenon and outside and chuk to reverse the inside. A dovetail recess would
also, I expect, be out of round after drying. My guess is that this would be
harder to re-true than a tenon as you don't have the nice convenient depression
from the tailstock left from the roughing operation.

Kip Powers
Rogers, AR