Hugo Kornelis explains that both seeks and scans exist for a good reason:
Fact: You should never blindly trust anything you find on the internet. And right now, you are reading the internet. So why should you trust this?
You shouldn’t. At least, not blindly. You should verify. And what better way to verify then through demos!
There went my strategy of blindly trusting Hugo.
The rule of thumb I have heard and go by is, if you’re retrieving less than 1/2 of 1% of data, a seek is the best route. If you’re returning more than 20% of data, a scan is the best route. In between is the “it depends” zone, where either could potentially be better. But please do read Hugo’s post—it’s an important one for query tuners.