that depents on the mistake. some people are trying to cheat from adsense (for example , think this some sites are putting only adsense ads on a page without any content). i report them to the adsense
i have seen a purely obvious MFA site using arbitrage. i saw their adsense ad, click on it and found only ad blocks and a line of links of their other webpages. clicking on the other webpages shows the exact same thing. i didn't report though, google should have done their job and filter their adwords user, which i think they are doing right now.
I only report if I see someone trying to exchange clicks or asking for clicks because this is stealing from the webmaster who are paying for the ads. For ad placement or any other thing, it is Google problem to police their own ad system not mine. I would rather report Google for allowing a site to break all the rules until they reach payout then ban them before paying. I can almost guarantee you that Google does not go back all that time and pay back the advertisers who paid for the clicks.
i report sites that has obvious policy violations like pornographic theme and invalid clicks and about 90 % of the sites i reported either had their ads gone or their site wiped off in roughly a week.
Well it depends...certain sites provide good information and usually if they put images next to other ads, that won't bother me much. However, there are others which overdo it and even put something like "click here to find out", then I'll report it.
some people forget to use the allowed sites in their options and then their competitors place their ads on bad websites just to try and get them baned. often times these people post the bad sites on forums like dp in hopes that more reports will result in a ban for their competitors whos ads they copied and put on the bad sites its funny how people are so ready to report people too when they do this.. its way more common then people think therefore id NEVER report anyone ESPECIALY if some one posted the site on a forum and said "look how bad this site is help report".. because it may be a new adsense users account and some one trying to get them baned by deception.. and by reporting you may actually be helping the real bad guy and hurting an innocent.