Well, realistically things are only like a couple CPU ticks faster... Your right, things wouldn't be noticeable. However, If you were to run the queries billions of times over you would be saving time.
MYSQL_ASSOC means mysql_fetch_array() will NOT index the array...
$q = "select * from something";
print_r(mysql_fetch_array($q)); would return
Array(0 => 1, 1 => "User", 2 => "password", id => 1, username => "User", password => "password");
print_r(mysql_fetch_array($q,MYSQL_ASSOC)); would return
Array(id => 1, username => "User", password => "password");
mysql_fetch_assoc() I believe is the same as using mysql_fetch_array(query,MYSQL_ASSOC)