one there is never a 90 wall and won't ever be. I never cope because it take too long and you can compound angles of six faster than you can cope one angle.
I rely on good putty and filler to take care of all my differences. Therefore, with this assurance and experience with products, I cut all angles, inside and outside assuming all are 90. then I compensate with fillers and light sanding.
these imperfections can be hidden very easily and no one will know except for you.
One thing, I never work with long stock. I go in and do one corner at a time. I have one foot on one side and one foot on the other. On the ends of these fitted corners I cut the molding at a 20 degree angle so that my straight stock will fit to this. Once all angles are make through out the job, then I come in and insert my straight stock to fit to the corners.
Nothing is worse than cutting a compound cut or non compound cut and working with 8 or 10 or 12 feet of stock. This long stock just makes your cut off more due to the movement of the stock on the rack.