So does relevance. The search engines see links that are tightly focused and exactly on theme for the site's topic. Playing games with nofollow "might" conserve a bit of PR, but on the other hand I'd lose credit for the relevance of those links. I'd rather have the relevance.
PR doesn't leak from your site and you cannot conserve it. This is a misunderstanding of PR. Linking to spammy sites can get you penalized and get you PR 0 so it may appear that it leaked but it's really because of a penalty. You can actually get better rankings by linking to good sites as long as they aren't your competitors and they are helpful resources to link to.