With adsense its not about filling your pages with the code, its about embedding it neatly into your content so your users will click it. If i see a page will not look good with adsense on it I simply don't put it in.

Following on from what West said its a good idea to put ads on pages that have a high view rate so you are likely to get a good CTR.

However I like to have at least one ad unit on your homepage just a basic text links maybe, but definitely not overwhelm your page or you will lose your visitors.

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