Johannes -
Untagged = 1 VLAN per interface
Tagged = 802.1q tagging (multiple VLANS on an interface), or a TRUNKED interface in Cisco terms.
There isn't really a right or wrong. The interfaces work either way. Personally, I like to have all my VLANS in one group running to a Trunk (f5 terminology) on the LTM (multiple interfaces bonded together using LACP) and to different back end switches if your environment can support it. That way if one interface goes down, you don't lose the VLANs and cause a fail-over on the LTM if you are set up that way.
Hope this helps.