The only issue with PR0 sites is if they have been banned by Google. Check to to see that their site comes up when you type in the domain name, if not, they might have a penalty. Linking out to sites with penalties (bad neighbors) is not good for your site.
PR is cumulative, so linking to lower PR sites is not a bad thing, people generally search out higher PR sites to link with since they provide much more benefit to you. A single PR5 page pointing to your site theoretically provides about as much benefit as 500+ PR1 pages pointing to your site.
Lastly, kwblue has a good point. Those sites that have good content and are pursuing a linking campaign won’t remain PR0 sites for long and you will likely get longer term benefits from them.