you can draw curves in any view you want but, they best done in orthographic ones.. you if you found a nice angle that you want to draw a curve in the perspective.. you can make the perspective orthographic by selectin' your perspective camera, or any free camera and pop up the attribute editor.. then slide down to Orthographic Views and tick the Orthographic option box.. now your perspective camera is a orthographic one, untick to unmake orthographic view.. :p