It all depends on the quality of the staff, whether you're shopping at GNC, Wal-Mart or wherever. If you employ commissioned salespeople, there could be the tendency to have them sell the items that make them the most commission. That being said, if you employ commissioned salespeople who care about their job, and care about helping their customers, you will usually get the right advice, and receive the correct product for your needs.
Remember this - if you are a commissioned salesperson, and you sell your customer the right product, regardless of your commission, you will likely get repeat business from that customer. If you sell your customer the wrong product just to make the commission, and this product does not measure up, your customer may go elsewhere.