A very simple technique that works for us is to rip a post lengthwise, in this case with a saw bevel of 22.5 degrees. Then flip over one of the halfs end to end and glue the two ripped edges together (the glue makes it slippery and the "halves" will start to warp immediately so plan on using fastners to hold them together while the glue sets up. This allows the surface of the post presented to the beams to be a simple, traditional plumb, square surface for simple joinery. We use this in conjunction with our splines and it has saved untold hours in detailing, cutting, and head scratching.

Alternatively, if the posts are big enough then the beams could be housed into the sides of the posts, the 22.5 degrees would be cut off the end of the beams and the mortise would slice thru the post at the same angle. Remember, tenon grain should be straight, a smaller extension of the beam so that the wood fibers remain continuous thru the tenon and into the main timber.