To make nailing in the crown a little easier, and if you have a table saw, lay a sample piece of your crown in a framing square so the edges that will fit against the wall are flush to the inside angle of the square and get the dimensions of the triangular opening. Subtract a quarter inch from each and set up your table saw so you can cut a triangle to these dimensions. The angle varies on different styles of crown but it will probably be around 35 to 45 degrees. Rip some 2x4s to the angled shape. Then find your studs and mark the locations. Nail the 2x4s to the walls so they fill up the hole you would have behind the crown. Now you can nail the crown anywhere and don't have to worry about holding it up in position and nailing into a stud at the same time.
Definately cope inside corners. You can cut 2 pieces of the crown flush to the wall for 2 opposite walls so you get to make 4 square cuts and 4 angled and coped cuts in one room. Cut the crown a little long and back cut the copes about 5 degrees. The backcut angle and a little sanding should give you perfect joints. You will be able to live with a little outward bow in the middle of the crown when both ends are perfect as nailing to the 2x4 backer will let the extra length be translated into short bows that will become invisible. Nail the ends, then the center (half the length of the run), then quarters of the length of the run, then eights of the length of the run.
Good luck--and you can do a perfect job!!! Henry in MI