WHEN TO USE WASHERS DURING AN ENGINE BUILD

I would never use a lock washer inside an engine, such as an oil pump bolt. It has an open end, which can cause it to break & occasionally they do. If the fastener is properly torqued, it should not loosen, negating the need for a lock washer.
Plain washers used with steel or c/iron I believe is mostly unnecessary. If the fastener has a lot clearance in the hole [ eg, 3/8" bolt in a 1/2" hole ] a washer would be a good idea to better spread the load.
On alum, use a washer.