I do take a look at it, but conversion rate isn't my main factor. I think it is too much of a wild card. I figure that low conversions could be due to anything. Perhaps the program with high conversions has affiliates who are better at preselling.
The two main ones for me are commission rate and if they have a datafeed. I like to be able to customize and integrate their products into my site instead of just tossing a banner up, and I can do this better witha datafeed.