My experience has been, if your Citrix traffic shares the media with other traffic, enough so that any congestion forms, QoS is a must to insure the best possible performance.
Do not design QoS on paper, implement the network in the simplest manner first, and monitor it for a while.
Check that traffic volumes and patterns are the ones that you have dimensioned the circuits for.
Check that you can make huge copies across, do voip, whatever else without disturbing the core business.
Check that there are no errors anywhere, redundancy is working as expected, and most important, everyone is happy.
Also secure it as necessary.
Measure, with netflow, span port or whatever you like, traffic volumes per application, that will be your baseline for eventual QoS configuration.