If you are not Geotargeting your website visitors based on you are really missing the boat. Not only could you be losing revenue but you are probably going against the TOS by showing these ads to people not in the USA.
YPN and other contextual advertisers stipulate you must have US visitors if you are showing there ads.
I just started doing geo targeting about 3 months ago and have seen a 25% increase in revenue for those slots.
I used to show people from the UK my USA CPA affiliates but now they see offers they can actually use and of course, I can get paid 😉
There are programs like phpAdsNew that will handle this for you.