The link building process can be long and mundane. If you know what the misconceptions to link building are, you can plan your time and resources accordingly. Add in some creativity and you will go far in link building.
In the decision of which links to pick portion of the link building process, taking into consideration Google PageRank and relevance is natural. They are both important, but not decision make or breakers in the process.
Google PR, or PageRank, is the ranking that Google gives a specific page that it crawls. PR is useful in a general sense for showing a site's range and scope. If you try to decipher the differences that make a PR3 and a PR4 you will go batty, and that is not conducive to link building. Additionally, PR is not a good representation of Google's system, since a PR0 link can prove equally as useful as one from a PR3 site.
The common way of thinking is that the most valuable links come from the ones with similarly themed, relevant pages. Although true, taking that for face value can encourage missed opportunities. The best links can be found on like themed sites, or within the same industry, but links that aren't can be useful, as well. Use common sense, but just because a link is not the same theme as yours, don't bypass it.
Provided you can procure your instinctive ways to value inbound links, sans relying on PR, increasing links to your site will come naturally. It will also prove to be more natural in your link profile.