At what point is it not worth submitting to a directory based on their google page rank? For example, a rank of 6 seems to be pretty good. What do you use for a cut off or does it really make a difference?

Is that the PR of the page the listings are on, or the homepage of the site? And how about whether they're direct links or redirected, and how the site is indexed in the engines?

that is pagerank the listing are on