My house was built in two thousand and four. Not a single wall in this building is perfectly square. I found this out when I tried to install new crown molding in the dining room last weekend.
I watched a video that said you just cut the corners at exactly forty five degrees on the saw. I set my heavy miter saw to the exact angle and made the cuts. I held the two pieces of expensive primed pine up to the ceiling. There was a massive half inch gap right in the middle of the joint. The actual corner of my room is closer to eighty eight degrees.
I tried to fill the huge gap with basic wood putty. The putty just shrank when it dried and cracked straight down the middle. I had to scrape it all out with a flathead screwdriver.
Now I use a cheap plastic angle finder before I make a single cut. It tells you the exact angle of the wall so you can adjust your saw. It saves me from ruining thirty dollar pieces of wood. I still end up using a little bit of painters caulk to hide the tiny seams, but at least the boards actually touch each other now.