If I have a website about nutrition and I want to add the site to a web directory under there Health section, is that considered a relevant link?
Also, if I do submit a link to these directories, should I be focusing on quantity or quality? Is it better to submit to a combination of sites with pr 9, 8, 7 or to a combination of sites with pr 9, 8, 7, 6, 5?
Since almost all directories are free, there's no harm in submitting to all of them. I think big PR web directories take a LOT of time to review and approve your site. In some cases, your link can take upto 6 months to be reviewed and might end up as being rejected The new directories dont have PR, but you are sure to get a link from them quickly. So its a trade-off between quality, quantity, time and links.
I am sorry, but I think you are a little off here if you think pr6 and pr5 are too low to focus on. Getting anything above 5 might prove to be a challenge. As far as directories go, the page your link will be on will usually be 1 or 2, even if the dir itself is 7,8,9.