It's only worth it if it's a laptop with known heat issues (specially, gobs an gobs of TIM applied at the factory, which is usually the case but rarely the sole cause of heat issues), AND you're afraid to crack it open yourself.
Here's some more food for thought: if you're afraid of opening up the laptop yourself and poking around, whose to say the person replacing the TIM won't make a mistake either. A loose screw can lodge into a "safe" place for a while, making the unit pass all quality checks until it reaches the end user. Then, when the unit is bumped or turned a certain way, catastrophic failure.