An often overlooked feature of Google's Adsense is that you can set up a list of advertisers you don't want to appear on your sites (the Competitive Ad Filter List)
Filtering competitors, if you have any, is probably a good idea. You don't want someone on your site clicking on a ad that leads them to a competitor's, unless you're very positively sure you're better than them :)
Filtering what seems to be low paying advertisers and cheap keywords is probably a bad idea. Google is supposed to serve the best ad, so if it shows something more or less irrelevant about some free web-based RSS reader, well, it may just be that or a Public Service Ad. Blame your content, not Google :)
And it's probably a good idea to filter yourself out. While surfing on whois.sc I noticed that they were serving ads about… whois.sc. Not only they're potentially being charged for someone already knowing their site and clicking on it, but they're also preventing them from displaying another potentially better performing ad. To be honest, I think not displaying your own ads should even be an Adsense built-in feature.
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