PCWorld has a good write up on PC myths... but read carefully as their ratings seem to contradict the text at times.  They rate the responding to Spam myth as a level 4 out of 5 on the bogus meter but then include this quote:

"Knowing who to opt out from is key," says Schwartz. "Opting out of legitimate companies drops you off their lists, but when you do that with 'real' spammers, the results are unclear."

So, what is it?  A myth?  If the results are unclear can you be sure?

A summary of the myths (go to the article for full details).

Busting the Biggest PC Myths
We expose the bad advice that wastes your time and money.

1. Magnets zap your data.
2. Using a cell phone on a plane interferes with the navigation and communications systems of the aircraft.
3. If you don't 'stop' a USB device before unplugging it from a PC, you'll screw things up.
4. Cookies track everything you do on the Internet.
5. Windows' Japanese edition uses haiku error messages.
6. Terrible things happen if you turn off your PC without shutting down Windows.
7. Opting out of spam gets you even more spam.
8. Hackers can destroy data on your computer's hard drive.
9. Turning off your PC daily to save power shortens its life.
10. The government reads everyone's e-mail.
11. Saddam Hussein bought PlayStation 2 consoles to use in Iraq's weapons program.
12. DOS is dead.
13. Only a pricey surge protector can keep your devices safe.
14. If you don't periodically run your laptop batteries down to zero, you'll lose battery life.
15. If you don't use an antistatic wrist strap while tinkering with a PC, you'll ruin hardware.