I am often asked what necklace to wear with different necklines, I have listed my suggestions below:  

Strapless:

An extremely versatile neckline. Choker, Y-drop, Pendant, Back Dangle or Lariat

Sweetheart:

With or without straps. Choker, Y-drop, Pendant, Back Dangle or Lariat

Round neck:

Choker or long y-drop.

Halter neck:

Y-drop or pendant but only on a low cut dress. If you have a high neckline, don't wear a necklace just earrings, bracelet and hairpiece.

Asymmetrical and Elizabeth/Queen Anne:

Don't wear a necklace just earrings, bracelet and hairpiece.

Square:

Choker or longish pendants.

V-Neck:

Choker, Y-drop, Pendant

Low Back:

Lariat or Back-dangle

Boat Neckline:

Choker

Off the Shoulder:

Choker, Y-drop, Pendant, Lariat or Back Dangle