A few ideas:

• Conside capsule zones instead of spheres as it will help you visualize your placement a little better and may give you the gradient of influence you're looking for.  (P8 and up)
• If you need to stay with spheres, just the display properties of your spheres to something more solid so you can better see how they're placed with respect to your mesh.   You may be having some issue with how the curve to the spheres are interacting and it will be easier to see your zones.  Just select the zone and go to Display -> Object Style  and try something like "hidden line".   
• Resort to multiple bones per piece that needs to curve.   This how I handle strings, ropes and long tails.   Check out easy-pose underground or philC's "All Tied Up."
• Resort to magnets including magnets parented to bones.
• Resort to weight-maps (P9 and up)

HTH,

-K