In my english campaign I recruited as much as I needed for my initial push into the north of england, without regard to how much it will actually cost. When I realized my expenses were going to outpace my income, I halted unit recruitment, and used what I had to expand. It took a while before I started recruitment again, but eventually as my military needs took priority, I started recruitment again. Usually this meant I recruited more than I needed at the time, but that just meant that I had the available forces to look to expanding once again.

If you're concerned that you're losing money on a stack of units, they are obviously not fighting enough. Send them out, and if they don't sack something, they don't come home


That occured on my crusade army to Jerusalem. It took forever to get through the Alps, and by the time the crusade target was taken, my troops only made it to Venice. Already weakened by huge losses due to desertion, Venice (held by rebels) looked to be a mighty fine target. Well, even after taking Venice, and then being threatened by a much larger Sicilian army, whereby I made the decision to completly sack the city and give it to Sicily, they still didn't return home. The stack was in time and position to head to Marseille as I was driving back Milan, and then later he personally lead the attack on Ajuccio and Cagliari, all with his original troops some hired mercenaries, and some minimal reinforcements. I think he was regarded as one of my greatest generals.