You should always have a torque wrench for working on a bike. As
@Tars Tarkas says its not for grunt work, but for making final adjustments. Almost everything has a torque spec on a bike, even if its a simple water bottle boss that has nothing more than the general torque spec that goes with a stainless M5 socket. You'd be surprised how low the torque number is to securely tighten but not stretch the threads on an M5 or an M6 bolt, and its just as important to use a torque wrench to avoid overtightening as it is to get something properly tight.
For a bike, a small 1/4" drive torque wrench that maxes out at about 10 Nm will do every job on the bike except the axle and the crankarms (the latter being a big deal to pay attention to as they often loosen).