Do you ignore blogs and websites with a PR of under 3? You could be making a big mistake. Where do you think the PR 6 and up blogs came from?
It is not uncommon to get a little arrogant regarding where your links are coming from. So often, it is easy to focus on chasing down links from PR6 sites that the fact that those sites were once starting out from scratch is forgotten. A link from a PR2 site may not blow you away, but it still has value and that value will likely only increase.
It is undeniable that links from sites with a strong PR are the most valuable, but they are also the hardest to obtain. On the other hand, incoming links from PR2 or PR3 sites carry less value, but they are usually also easier to obtain. What you need to search for is balance when building your incoming links.
When you start looking for links. Start by putting about 1/3 of your time into going through low PR sites. Rather than waste time with old sites that are poorly maintained and may even appear abandoned. Look for new sites. Read some of their content, check their layout and determine whether or not you believe they have staying power. If you think the site is quality, go for a link. Right now they may be a PR2 or even PR0, but if they have a sound site and you can get a relevant link take it. When the site grows and becomes a PR6, that link you got on the ground floor for free may be something that would cost you money later.
There is a time when you actually want to turn down links. Link farms are useless for the most part and provide negligible benefit at best. Links from sites that have no relevance are not going to count against you, and sometimes they will help a little bit, but they are not worth the energy to actively seek out. The one type of site you want to avoid at all costs is any site using illegal SEO tactics. Links placed on those sites may actually come back to damage your placement.