Because I was already registered at other SimpleMachine forums, I encountered cookie problems.
I think it's best to add the cookie-name option at the installation so that people don't have to delete their cookies or login with another browser in order to fix the problem.
You don't always get the same cookie name.
SMF uses a number with the cookie eg SMFCookie270
The number isn't random, but passed on this
(from install.php)
'cookiename' => 'SMFCookie' . abs(crc32($_POST['db_name'] . preg_replace('~[^A-Za-z0-9_$]~', '', $_POST['db_prefix'])) % 1000),
So it might be unlikely that you'll get too many forums with the same cookie value.