Just remember that every resource you devote to the happiness of your people is something you could have devoted to your military forces. The tolerance of your peasants can be elevated by researching certain skill, your general's buffs and building certain structures. Just be sure to check in every turn and make sure it hasn't progressed into red.
Being the cautious sort we chose to leave it in the green, that said it is possible to get away with yellow for a time. How far you're willing to push this is up to you. Green means satisfied, yellow means annoyed, and red indicates that your peasants are ready to rebel. The level of a province's dissatisfaction with their current tax rate can be gauged by their color on the empire map that accompanies the finance management window. Raise them too high and your provinces will rebel, causing you all sorts of trouble. That said, raising taxes also comes with the side effect of angering your subjects. Raising taxes obviously yields more money to help maintain your armies, make repairs and upgrades to your infrastructure, and altogether survive. While managing your finances you'll have the option of raising or lowering your tax rate. The biggest bit of this is managing your taxes. What this mainly amounts to is keeping your subjects happy while at the same time milking them for war funds. That being said, Shogun 2 isn't just about duking it out with the daimyo next door. During the course of your campaign in Shogun 2 you'll undoubtedly conquer and connive your way into possession of some provinces.