I've never bothered to switch on the GEO targeting but today I logged into webmaster tools with the intent of switching it on for the US. Then I thought about how this might impact on my subdomains which are "country" specific. So what do I do? Switch it on and hope for the best? Leave it alone? Or maybe setup each subdomain in webmaster tools and assign each country? Consider travel which is not my game but it's a good example, when you make that country specific association then in theory your limiting your search exposure which would be less than ideal.