The Rule of Making Production Plan

After getting order from customers, the factories need to make the production plan.
What is the rule to make production plan?
I think that to make the production cost as low as possible is the rule to make the production plan. In this way, the factories can reduce the cost, and become more competitive.
Usually, customer’s order is not even, sometimes more, sometimes less. If the factories make the production plan according the order, sometimes workers need to work extra hours to finish the orders, sometime workers have not enough work to do.
If the factories make the production plan in a even way, when orders are less, workers work full load, and overstock products. When orders are more, the factories can delivery these products on hand, and workers need not to work extra hours. In this way, in the long time, the production cost is low.