Signing helps to prevent fraud. But if someone wants to sell counterfiet arrowheads, it's not hard to do. Many modern arrowheads coming out of India and Mexico, for example, are routinely sold as authentic. Fortunately, collectors are getting better and better at spotting items that are modern made. If you want to be a serious collector, however, you'll probably want to find 90% of your collection yourself: surface hunting or digging.
Signing can also add value to a reproduction if the artist is well known.