V-necks are not all created equal. Some are up high and straight, some are low and curved, and lots of others are somewhere in between the two. I think that all of the v-neck t-shirts out there have a place in the closet, but there are a few guidelines to follow:
1. Know what is too low: Men don't really need to enlighten the general population as to the landscape of their entire chest (so wear them layered with other knits), and women should follow this same rule (the only exception is when you are in a nightclub and all other body parts are appropriately covered).
2. Landscaping is key: Men should have their chest hair trimmed in order to avoid a tufted look, and women should be sure to conceal any splotchiness or acne on the decolletage for a clean look.
3. Less is more: When in doubt go for a higher v-neck in order to avoid having to consider rules one and two.