I use guard mode and fire at will to make sure the men get their volleys off without charging prematurely. Once melee begins I turn off guard if they seem to be winning because I want them to press the enemy not return to their spot.

And yes, fire at will can be dangerous so you need to limit the chances of friendly fire kills by turning it off for some units near the action. Shogun used to stop opportunity fire into melees unless you specifically ordered the attack but Rome does not.