I think conversion ratios should not be what affiliates use to sign up to a program, but it can be a useful contributing factor. We have used conversion ratios in our newsletters, but we substantiate our claims by showing an actual screenshot, which I do think gives a bit more credibility to the claim. It would do nobody any good to lie about the numbers because affiliates are going to find out once they sign up and if they don't achieve close to the numbers you advertise they won't be sending traffic for long anyways.

However, I think the 'conversion ratios' should only be a small part of the bigger picture when you look at an affiliate program to promote. You should also be looking at the design, does it have exclusive content, what kind of sales tool does the affiliate program offer webmasters, do they have good support, our they trustworthy and other factors.

Cheers,
DannyZ