I personally don't do a lot of retraining, rather I strive for building units in specialized provinces. Of course, earlier on your units are unbuffed, but so what of it?

I play on expert and I don't find it too easy or too hard to work with that. I try to dedicate provinces for various features, and as the empire grows, of course you may have some duplication but if you need to churn out men you can do it If they are unfinished uber units, don't worry, you'll live. Tatics is more often what carries the day, not supermen