I have a document from Professional Services called "Opening Balances Best Practices".  In one section it says...

The Inventory Adjustment Worksheet should be used for inventory adjustments ONLY when using average costing.  FIFO and LIFO costing requires that you adjust the inventory counts in dated “layers” which reflect the date of purchase of each item and its price.

Some FIFO and LIFO customers have successfully used the Inventory Adjustment Worksheet to enter their initial Quantity On Hand values, but have done so by sacrificing their date and purchase history “layers”.  Instead, they have chosen to use average cost for the current Quantity On Hand, and use LIFO or FIFO going forward.