In general, it should not hurt to use the google_ad_section_start tags. They can help Google to identify the relevant content on your pages. Note, even if you include these tags around everything unimportnat, it does not mean Google won't see the stuff. Google will still see everything, these tags just place/remove emphasis on particular sections of your pages.
The following is the proper syntax for the google_as_section tag:
<!-- google_ad_section_start(weight=ignore) -->
text that you want to deemphasise from Google bot
<!-- google_ad_section_end(weight=ignore) -->
Note, because this is enclosed in the <!-- --> HTML comment characters, it will be disregarded for display purposes by any browser.