We've had this debate before, with the conclusion that it doesn't really matter. The compiler will catch all errors except for a special case with "using namespace std" where undefined behaviour is invoked if you forget to include a file under certain conditions.
"using namespace std" is perfectly legitimate (especially for the example programs often posted here).

But you must remember to never use any using-declaration within globals scope of header files, because that will affect every file that includes the header.

Quote Originally Posted by Mr.OC
Using the std:: prefix follows the standards a bit more, I'd imagine.
There are no degrees of following the standard. Either you do, or you don't. Every variant does.