Do I need more than one cost category?

You can certainly run your business using only one cost category. That's all you need to set up item sites and track inventory movements in the general ledger. If you want more detailed inventory accounting, though, you should consider implementing multiple cost categories. For example, you might have a cost category for finished goods and a different one for raw materials. In this way you can track particular item types or groups by different accounts—thus leading to more detailed financial reporting.