When you open the ports to the public internet the security of your server relies on that of the application behind the corresponding port. So if you have an application running on 443 that is vulnerable to some sort of an attack then you will now have opened up a vulnerability to the internet. However; I'm assuming that if you are about to use that port for SSL you are confident there are no security vulnerabilities in the software running on that port.
To put it simply, opening that port before you have the SSL cert in place does not create any more vulnerabilities that what would be there when you did have the SSL cert besides the possibility of a MITM on a client using the web application.