I consider myself a pretty good finish carpenter. I,ve done enough. When I do base ,I use stock to fit wall to wall. 12' wall-12' piece. I always back cut and cope. Two 45 degrees cuts are certainly OK but I think coping is a better fit. 95% of the time the fit is workable. Very seldom does a corner fit perfectly. The only time you try to get perfect corners is when the material is stained.Then you still have to use putty. Nothing wrong with that. You have to fill nail holes anyway. Don't be so precise,the job will go faster. Ron J.