Using isnull() in joins

Let's say I have 2 tables - PurchaseOrderLine and PurchaseOrderLineAllocation.They both have a field named OrderLineID, but only 1 of them will ever be populated.

I need to join a 3rd table (OrderLine) to a 4th table (StockReceiptLine) via one of those 2 tables.

As it stands, the path would be:

1. OrderLine.OrderLineID - PurchaseOrderLine.OrderLineID
   
2. PurchaseOrderLine.PurchaseOrderLineID - StockReceiptLine.PurchaseOrderLineID
   

or

1. OrderLine.OrderLineID - PurchaseOrderLineAllocation.OrderLineID
   
2. PurchaseOrderLineAllocation.PurchaseOrderLineID - StockReceiptLine.PurchaseOrderLineID
   

I vaguely remember hearing isnull stops the optimizer from using indexes properly - would using isnull as part of the joins cause any kind of performance issues? If so, what would be the preferable option in this situation?

FROMOrderLineolLEFTJOINPurchaseOrderLine...