The way I look at retraining, there is a cost: the time it takes to get the unit to the retraining area, and the turn taken to retrain an existing unit rather than produce a new one there. Hence, I do not retrain.

I simply develop my economy to the point where I can train new units every turn if I want/need to. I keep a constant line of troops moving from my training areas to my front lines. I merge older, attritioned units into new ones with the better upgrades after battles, bringing them back to full capacity, and then remove the remaining partial units to a province just behind the front lines--ready to move in after a battle to top of depleted units when they are needed again.