Norm:
I guess you'll get different answers from different people. And I’m not saying that daiku’s answer is wrong.
But I would say let the roof weight and the draw bore of the peg holes pull the plate down and remove the crown. Cut your mortises with the correct offset location from your reference face and rafter seats on the surface of the plate and do not use a chalk line.
Then in the assembly process, pull your plate down using the draw bore method, thus removing the 3/16" crown. With the rafter seats cut on the surface, correctly attach the rafters and roofing. The weight of the roofing will help hold down the crown.
If you're not sure if the relish will hold on the end of the post tenon, then pull down the plate first, by putting a come-a-long on the plate connected to the sill over or near the middle post and pull it down until the draw bore holes nearly line up, be sure to pull down straight and then pound your peg in.
Then release the come-a-long and continue your raising or release it later if you wish. Again the weight of the rafters and roof deck and roofing will add to removing the crown.
Hopefully others will add their opinion to this topic.
Good luck with your project. Jim