I don't like using peasants, just ruins my nice array of forces. I usually work out a set garrison for inner and border cities, then setup all my cities like that. In my Saxon game I finished last night it consisted of 3 Levy Spearmen-2 Hunters for inner cities, and 2 Levy Spearmen, 3 Saxon Keel, 3 Hunters, 2 Saxon Sea Raiders for border cities. It worked out pretty well too, considering the only time I ever lost a city was when the Franks and Lombardi filled it with spies and it rebelled on me.

Despite the costs i'd go with professional(non-peasant) garrisons everytime. It makes your cities easier to defend and easier to reinforce in the event of a big invasion when time is a factor(just by moving troops from inner cities to border or the like).