I'm somewhere between a 32c or a 34B, and I have learned recently to love long necklaces. I now own four and I'm even having fun combining them to wear together.
I like them to fall at a certain sweet spot, which, come to think of it, is right around my natural waist, which happens to be on the high side (an inch below my rib cage). If it falls too low, it only draws attention to my squishy midsection. Being short, this is why I had a mental aversion to long necklaces - most of them would hit exactly where I did not want them to. But, now I'm proof that even if you're only 5'0" you can find long necklaces just the right length.
However, I will say that sometimes the necklace will get "caught" around one of the "girls" - if you know what I mean. Here is the perfect picture to demonstrate - I believe the satin of this dress made the necklace want to slip and slide, because it did this ALL NIGHT:
http://s386.photobucket.com/al.....orm181.jpg
I find a collared shirt often helps the necklace stay put better:
http://s386.photobucket.com/al.....orm169.jpg
As does a cowl neck:
http://s386.photobucket.com/al.....orm170.jpg
A jacket can do this too.
I actually ended up knotting the necklace for this outfit, because there's something about this tighter-fitting shirt that led to the necklace want to wander in that annoying fashion:
http://s386.photobucket.com/al.....orm282.jpg
So I do find myself having to adjust here and there because that just looks weird. If you are very well endowed, and if your bra doesn't keep you, erm, separated, you may find the necklace gets stuck in between, instead of falling down in an elliptical shape getting squeezed together in one double-stranded line. Hopefully you can picture this and understand what I mean.
My experience is also that a heavier necklace will be more likely to stay put where you want it.