A basic of driving a steam engine is not to waste steam, since steam represents fuel, which in turn represents money. As you say, not having an accident is more important than wasting a bit of fuel (hence safety valves on steam boilers), but in everyday operation wasting fuel is a cardinal sin and something that is to be avoided. At one point I was involved in steam rail preservation, and the old-timers used to mourn the loss of engine driving/firing skills whenever engines sat at the platform with the safety valves blowing.

The ship command structure was a form of information hiding, whereby the commander only worried about the speed, whilst the engineer worried about how to efficiently deliver the requested speed.