Sorry in advance about the verbosity. I am typing this in a hurry. It might have found a more concise way of expressing myself if I had more time.
Yes, that is the most common assumption. The whole American (and, since we monkey everything after the Americans, European too) production system is based on that. However, there was once a company that stopped assuming and started measuring. They found that, perhaps counter-intuitively, utilizing your production line close to full capacity is detrimental to its throughput. Any small variation at any link in the chain causes huge dropouts in productivity. Their findings were one of the bases for the Toyota Production System, or TPS as it has become commonly known.