Edit: OK, I got it working! Here's my steps for future Methodists who find this post:
The goal here is to hide Qty, Rate and Amount values for line items that don't actually need them - Our Sales Orders will have lines containing notes or shipment information in the Line Item Description field, and having the Qty set to 1 and the rate/amount set to $0.00 for those lines is distracting clutter.
If you click on any object in your report designer and expand the options menu (the little white arrow in the corner of the object), you'll see a "Formatting Rules" field. Click the "..." button to open the Formatting Rules Sheet Editor.
1. For Data Member in the Data section, I selected "SalesOrder.SalesOrderLine.SalesOrder". For Data Source I selected "dataSet1". These matched the selections for the DetailReport section that prints line items in the report, but I'm not sure if this step is necessary or correct.
2. For the Format Rule's condition, I put " IsNullOrEmpty([Item]) == True " . In other words, if there is no value in a line's Item field...
3. Originally I had set "Visible" to No, but since I have my line item fields in table cells, it made the cell borders disappear as well. So my current version has "Visible" set as Yes, and the "Foreground Color" and "Background color" are both set to white, so the text is effectively hidden while the cell borders remain.
4. To apply this rule, I went to each field I wanted to hide, went into its "Formatting Rules" screen, and moved the created formatting rule from "Rules available in report" section to the "Rules applied to a control" section.