I always start building models starting from side wheel fender. From there i can see the main line countour which shape the model at the end. Also its due to georaphic factor since the wheels was one of the first main obstacles to encounter when want to shape out the design.
Unlike the traditional spline to face creation to complete a 3d object, i use the object morphing technique to produce a desirable object i want for example the fender start up with cutt-out circle object: stretched, resive and vertices displacement.