The way I see it, VLAN tagging on the MCD interface is there for small setups where the simplest instruction to the local network guy is, "create a new VLAN, and add that VLAN to all switchports as 'tagged'".
Then when the Mitel guy comes on site to install the phones, all the switchports are good to go for phones, and the voice controller is tagged at its end in the voice VLAN and is also good to go.
The setup I have always used is more like,
"create voice VLANs and add them as a tagged VLAN to all the Access switches, for the phones"
"assign me the IP address you want the phone controller on (should be a SERVICES VLAN, not a VOICE VLAN) and tell me which switchport to connect the controller to (which will be untagged in that VLAN)."