Proxying and redirection on exchange 2013

hi

Microsoft has a rather lengthy document about proxying and redirection in exchange 2010.

http://technet.microsoft.com/en-us/library/bb310763(v=exchg.141).aspx

It says if you have CAS servers that are not internet facing in other AD sites, then you need to set the externalURL values to $null for those CAS boxes

1. Does it apply to exchange 2013 as well (Setting the ExternalURL value to $null for non internet facing 2013 CAS)? There is nothing in that page that says anything about 2013

2. How about coexistence between exchange 2013 / 2010 when CAS2013 is put in the front?  Do you leave the 2010 ExternalURL values on non internet facing sites as $Null?

Thanks