You should start thinking about the CG when you first open the box regardless if it is a kit, scratch built, or an ARF. Besides the simple balance of the model, how the mass of the components are distributed affects the flight characteristics. The technical term is the polar moment of inertia, but in laymans terms having the weight farther away from the CG increases the polar moment around each axis. So it takes more force from the controls to start movement, and takes longer to start and stop movement.