Taking damage does not decrease a unit’s damage output. A near-dead Scorpion tank down to its last red bar of health still inflicts the same amount of DPS as a fully healthy Scorpion. So it makes sense to keep each unit engaged in combat as long as possible, pulling it back for healing only when its overall health dips into the red zone. Remember that Nightingale support craft (if available) can hasten the healing of any UNSC unit, vehicle or infantry. (Engineers do the same for the Banished.)
Repairing multiple existing units with Restoration Drones or a Nightingale support craft is cheaper than training new units. Keep a close eye on your key units and base structures during every engagement. If the enemy seems to be pounding on a particular target, run your cursor over it to check its health from time to time.
If a unit’s health bar is dropping precipitously, pull it back out of the firefight and park it behind healthy units. If the target taking heavy damage is a structure, consider an immediate hit of Restoration Drones. Do this right in the heat of battle if necessary. Losing a structure is always costly, in terms of resources and lost production, so timely repair work can be crucial to success.