I believe 'untagged' is needed for outgoing traffic to vlan-unaware devices like a PC. The tag is removed before leaving for the end-device.
'Tagged' by contrast is leaving the tag untouched, which is required while the traffic circulates within the network, as e.g. trunk ports connecting two network-devices.
In my example, traffic arrives with vlan4 tag on port 1 of the router, passed on with the tag to port 5 on the switch, which then hands it to port 3 or 4. If either of the previous ports were untagged, the traffic could not be directed to port 3/4. They in turn have to remove it, as the end device does not know what to do with it. And the other way round backwards from PC to router.
It is my understanding, using my words. Hope the experts forgive me for using unskilled word
When you have at least a tagged vlan, it means the other end of the cable also supports tagged frames. Personally I don't see the sense of mixing them. For inexperienced users it is another potential pitfall. If you feel confident go ahead and use it.