How to make new Email Address Policy lowest priority?

We are running Exchange 2007 Service Pack 2 on a Windows 2008 server.  We currently have one email address policy, "Default Policy," which uses the alias followed by @domain.com.  We now are going to add a second smtp address in the First.Last@domain.com format.  I'm going to use the Exchange Command Shell to create and apply the policy, in order to filter the recipients of the policy.  I want to make sure that our Default policy remains as the Primary SMTP address on all accounts, and the new policy is at a lower priority.   Do I need to use the 'Priority' parameter in my command, and if so, what should the priority be for the new policy to be sure it doesn't become the Primary address?

Thanks in advance!