Its pretty simple. If you have any errors that would hinder the search bots from crawling your site, or any type of usability errors, then you should take the time to fix it. Anything else is waste of time and it would behoove you to spend that time building links or something that will bear fruit.
The problem is many of us in this business are not programers, so spending hours upon hours to fix something just so some website can give you a 100% score is silly, unless the errors are causing any genuine problems with your visitors or the bots. But I understand how some people are ficklers about things being right, and if it gives you peace of mind to fix all these errors, spend a nominal fee get er done. But I personally don’t sweat it.