Section targeting doesn't automatically work. Google only pays any attention to it if the system is having trouble putting your page in context, and with most sites, that isn't the case, so adding the tags won't make any difference. If you were getting targeted ads before, it's probable that the tags will be ignored anyway.
I used it effectively on my home page. I have an adlinks unit on that page, and some of the links were connected to content that didn't reflect the main subject of the site. So I use the "ignore" code and the adlinks definitely improved.