It's all wrong if your name happens to be Will.
"Fire at will" was coined because the long reload times of early firearms left a long gap between the time that the faster reloaders were ready to fire again and the time that the slower men were done reloading. That meant that more fire going downrange was possible by allowing each man to fire at his own pace. On the other hand, controlled volleys were preferred in other situations, by withholding fire until the charging enemy was getting uncomfortably close, in order to get the most hits with inherently accurate firearms. Wait too long, and the survivors of the volley would be upon your men before they could switch to melee weapons. Fire too soon and most of your shots miss (as in 95% or more).
The phrase "Ready, AIM, Fire" was not yet in use, because aim was relatively pointless with a weapon that typically scattered its rounds by more than a yard from the aim point at 100 paces, and a man with equipment could cross that distance in 10-15 seconds, far too quickly to reload and fire again.