Forum Moderators: open
Example: If you have a link to your site from a site which spends millions in advertising, but not in Google at all, it does nothing for your rank in Google. If you have a site that has never had a unique hit from a browser, but is ranked highly in Google, it will assist your rank.
Don't worry about how much traffic that site gets, how high its PageRank is, etc.
Bottom line is: if you feel the site you are linking to would be valuable and informative to your site's visitors, then go ahead and link to it.
Anyone who has been on the internet for at least 6 months knows the difference between sites that are "good" and sites that are "bad".
Just use your best judgement.
*and remember, this is the 'Search Engine Promotion' forum, not the 'Google Engine Promotion' forum. Page Rank means nothing anywhere else but Google*