Since I figure ads help bring me the content I want to see, I won't block friendly ads. I will, however block pop ups, animations, misrepresentations, and other offensive material.
And if an advertiser is serving from an offensive site, it might inadvertantly get blocked as well.
By the way - I also hit the clicker when a cable channel hits me with pop up ads.
If you don't have a firewall running already, I recommend Agnitum Outpost Firewall Pro (www.agnitum.com). If you remember AtGuard from the Win98/2000 days, Outpost does it all and more. It will block any sort of content and/or ad string based on preset rules or prompt you to create rules as it encounters junk.
I use Netscape 7.2 (ver 8 is a train wreck) and Outpost and am rock solid.
PS - check out the Blockpost extension for Outpost also! :)