WEll, I've gone through my cities and realized I should use peasants in certain circumstances.

First and foremost, in the larger cities. 10 town militia don't even give you 50% public order bonus if your town is over 25,000. So the extra 40 men in each peasant unit come in very handy, and cost less too. I'm definitely going to use peasants in cities that won't be attacked.

Peasants are also useful in cities that have small garrisons that they need to build up quickly. Again, the extra 40 men improve public order faster, cost less and slow the population growth more. Plus, if there is rioting, they are cheaper to retrain if they take casualties. If you have just taken a city that is very rebellious, peasants are the way to go for sure.

Thanks for the tips!