"Nickel-Free" vs. "Hypo-Allergenic"
The confusing thing is that there is currently no regulation in the U.S. which governs exactly how these terms can be used. “Nickel-Free” may still contain a small amount of nickel. The amount of nickel is such a truly tiny amount that even most allergy suffers will not get a rash. Surgical Stainless Steel would fit this category, and is considered safe for most people. If you want to avoid metals that may contain even a small amount of nickel, stick with niobium or titanium, or alloys with known content, such as brass or bronze (though double-check about any lead content just to be sure).
As for the label, “Hypo-Allergenic,” just don't trust it. It doesn’t really mean anything. It technically means “less allergenic,” which is not much of a guarantee. (Less allergenic than what?) By the way, that stuff called “Fashion Jewelry” in the stores? Well if it doesn’t say what’s in it, assume it contains nickel, especially if it’s cheap.