From lower PR sites or PR 0 sites you will get benefits of anchor text only. No any negative effect on your site.
Even a PR1 link will help, but a PR6 is better and there is one poster here whom says for $5 a month she can get you 30 or more PR6 links with a few PR8's or PR9's. I don't know why I have not tried her out yet. I have meaning too, in fact I will do it today and give it a review after a month or two and let you know how it worked.
If you choose low PR web to backlink,you'd better know how many backlink it have,because you understood this: maybe you can have a backlink from a web which has a high PR,but it has a lot of backlinks ,you also could not have a high PR,and if a low PR web has not backlink,you link to him,the PR maybe could improve. Hope you can understand my words.
Using sites with lower PR makes your links look more natural. Think about it - if all of your links are only PR5 and above, it would smell of cheat.