Well it has varied for various Windows versions:
http://blogs.msdn.com/wer/pages/faq.aspx#weronpc
But I believe that both places may be used.
On my Win 7 machine I have have both locations, but the reports are different.
Those reports caused by errors in applications or services running as the “System” user, are all in the %ProgramData% path, but those reports caused by errors in applications running with “User” permissions are all in the current user’s local application data (%LocalAppData%) path.