[gasket]

Google isn't worried about your site. They will drop your ranking if they think that is for the greater good. If you are going to link without nofollow then you are trusting every site you link to never do anything that even vaguely looks spammy or like paid linking. If you link to a site that pays for some links (according to google's spam algos) then you will get 'panda slapped' and recovering from that is very hard.

Always use nofollow.

[grey-area]

It's amusing that Google is opposed to other people using paid links, when their main business is... paid links.

[AznHisoka]

But then they can turn around and say "Go ahead, Ban Google.com from your search engine because we have paid links". DOH! Can't win =(

[josephjrobison]

Do not always use nofollow.

https://support.google.com/webmasters/answer/96569?hl=en