mysql - mysql_real_escape_string in php, where to use it?

I just learned about the importance of
mysql_real_escape_string to avoid MySQL injections. So i used
this in my registration form in php. This is what i
got,

$password_string =
            mysql_real_escape_string($_POST['PASSWORD']);
$username =
            mysql_real_escape_string($_POST['USERNAME']);

$fname =
            mysql_real_escape_string($_POST['FNAME']);
$mname =
            mysql_real_escape_string($_POST['MNAME']);
$lname =
            mysql_real_escape_string($_POST['LNAME']);

Question:
Do i also need to use mysql_real_escape_string on forms that
only verifies entries in the database? or is it only useful if you are inserting data
into the database? I asked this because I also have a verification form wherein users
will input data to see if those records exist in the database. Do i still need the
mysql_real_escape_string for
security?

Thanks!