Do I really need to stud nail all my crown molding?
Crown moldings, or any corner trims really should be nailed to a stud, not simply into unbacked drywall. Without good support, the molding will warp out from the wall causing gaps in coverage  to open along the edges.

With real crown, there is a hollow gap behind the molding as only the top and bottom edge touch the wall. The wider the molding, typically the deeper the void behind them.  This gap results in having to use longer nails to reach the wall and support wood behind the sheetrock. 

One method that works well to make installing the molding easier is to first install a backer/nailer to the wall. Depending on the size of the hollow behind your crown, cut or rip some scrap board, plywood or even strapping, then nail it along the top edge of the wall against the ceiling. You can shoot as many nails as you want to assure the backer if firmly attached to the studs or wall plates, as it will never show anyway.  Now, when you go to nail your crown up, there will always be a solid wood backer anywhere you shoot a nail. You can also use shorter nails, or at least not have to use extra long ones. This method is also helpful when your walls are not perfectly flat and an extra nail is necessary to draw the molding in tighter to the wall in irregular areas.  Good Luck