To those who don't think a torque wrench is *needed*, you've obviously not worked on a car where the spark plugs have only a small number of threads, and the heads are aluminium. It's easy to strip the threads on this configuration so proper torque is vital.
This is not your father's Oldsmobile with cast-iron heads and the old "quarter-turn past snug" no longer applies.
*EDIT* I know this doesn't apply to all cars, but unless you're SURE you don't have a potential issue, it's better to use a torque wrench.