I didn't download my Google toolbar until about a month ago. Before then I was swapping links with anyone who was willing to do so. PR was not an issue. And I was steadily climbing in SERPs to #1 on the last update for some very competetive terms. Now when I look for link partners I have been getting bogged down analysing page rank from the prospective link partners. PLUS- The partners I had linked with previously, before I had the toolbar, seem to have a PR within 1 PR ( + or - ) of my PR. Coincidence? I think not.
Trade links with relevant sites when you can regardless of PR-this will raise PR of all linked sites. However check out a prospective link partner first to make sure the partner is not spamming Google