Don't overthink it.. any link that benefits your end user's experience is a relevant link. Don't worry about having a few irrelevant links as most sites have them. Just make sure to avoid schemes that generate a high volume of irrelevant links in a short period of time and you will be fine. Ask yourself, "does this site I am about to link with benefit my end user?" If the answer is yes, get the link always regardless of pagerank or other metrics. If you aren't sure, skip it and move on to the next opportunity.