I agree with SphinxnihpS:
It's really hard if the corners aren't right on 90% and the walls aren't perfectly plumb. But, buying a few extra feet of molding helps: have a couple of extra 2 or 3 foot long sections. On one end of each, cut inside corners, and if needed, on the other end of each, outside corners. (This works if you have a decent compound miter saw)
Check the fit in each of the corners. Write down any corners that you're not satisfied with the fit of. You can label the walls with pencil right behind where the crown molding is. (or keep track on paper.)
If you're not satisfied, then eyeballing it & splitting the distance, adjust the angles slightly on the compound miter saw & shave the slightest bit off each of your test pieces. You might have to experiment a few times to get it perfect. Since you're making two different cuts, make sure you write the first cut down!
You can either do this method one piece at a time, or keep track of all the cuts you need & do them all at once.