taffy,
i think the type of necklace does depend somewhat on the neckline. here are my general rules, although i do bend them from time to time.
v-necks
i either stick to shorter necklaces or necklaces that are a couple inches longer than the lowest point of the v. sometimes if i wear a necklace that is just slightly longer than the neckline, it falls inside the shirt, which is never good.
scoop necks
for these, i usually wear a shorter necklace that "fills the negative space". the neckline just seems like the perfect frame for them. pendant necklaces work especially well with scoopnecks.
turtlenecks
as long as there is no detail at the neckline (ruffles, tie-necks, etc.), i wear a necklace with them. i recommend long chains/pendants with these. nothing too close to the neck b/c it doesn't help elongate.
strapless
the possibilities are generally limitless on this type b/c there really is no defined neckline.
i hope that helps!
