I think this option should be possible to enable/disable in account settings but it will be useful especially when someone is posting his/her opinion in topics like "Poland - rating as country to live in."
I think this would be helpful and is almost necessary. I have just joined and I was confused about whether this site was supposed to be US centric, I would have gone ahead assumed so but it said "global politics" at the top. In some threads like "do you trust the police" the answers only seem to take the U.S. into account and ignore the global content.
For the lack of a better option, i'm voting yes.
The full answer i'd like to give, though, is that it should be an option you can turn on and off. Let it be, on default, on, maybe?
I would most definitely be good to see the flag IF the user wants us to. Let's not forget this is an opinion website, and some countries have, erm, stricter policies regarding free speech on internet. Ex: I wouldn't want a cool user to get in trouble because he is talking against his regime.
But it would be very interesting to know where people are from, it adds an insight to their opinion. It could also be useful for graphs, comparing what people vote for according to their location. Ex: An Australian citizen might not have the same views as a US citizen on gun control.