In British service, it didn't quite work like that: inaccurate fire or ammo wastage were secondary concerns. Doctrinally, the idea was to keep an "ammo reserve" for when the enemy got close.
It was part of a larger body of training, involving volley sights and range estimation in an age when machine guns were poorly understood and/or in short supply. Imagine you're in a trench with the enemy charging toward you. They're 1200 meters away: even double-timing it, they'll still take around six minutes to arrive, loaded down with all their stuff. Your officers' and NCOs' job in that case was to know how to estimate the range, call it out, and order you to fire. Your job was to keep adjusting your volley sight before patiently responding to their orders. All of this was indirect fire.
There was plenty of time to load individual rounds. In fact, the drill was such that you'd probably know how many rounds you'd expect to have to load before the close fight began; you'd know that Sergeant Smith or Lieutenant Jones preferred to engage the enemy at 1000 yards or so, then hit them again every 150 yards until 550, then every 100 until 250, then directly from your magazines, "firing at will."
So you'd load your ten rounds in your SMLE, engage the mag cutoff, then take six or seven loose rounds and lay them on your sandbag or put them in your pocket. Then? You'd just open your ears, look at the blurred mass of Huns 1200 meters off, and wait patiently. When told to fire at will, you'd disengage the mag cutoff, flip the volley sights back down, move your sight picture to the actual sights, and let fly.