I spend almost no time managing my AdWords account now that I've got it running the way I want to. Using conversion optimizer my conversion rates are in line with my expectations. As buckworks suggests, I do go in an spot check to remove sites that are not a good fit and tweak for those that are.
Not all websites accept direct advertisers.
Managing those relationships that do on an ongoing basis requires continued attention. For some people the incremental margin might not be worth that additional effort.
Part of my advertising effort is market-share and mind-share driven -- not purely a conversion metric. AdWords provides a great opportunity for me to have my ads in front of an audience that I might never otherwise come in contact with.