Best Practice for internal IP address?

I have a very standard setup for my company - a Cisco router facing
the Internet, connected to a Firewall appliance with 2 zones, one a
DMZ and the other an internal network.

My question is what IP address do I give my inner facing port on my
router - a routable or non-routable one? If routable, I can telnet in
to it from the public Internet. However, I have read of best
practices that say you should never do this. I will have a VPN
solution so I can VPN into my private network and telnet from there.

And if I do make it non-routable, is there any advantage to making the
IP address different from the internal network, or does this introduce
complexity without adding any benefits?