Which version of virtio-win should we use ?

Issue

• There are several versions of virtio-win we can get from Supplementary channel.

  virtio-win-1.8.0-4.el7.noarch.rpm      (included in 7.2)
  virtio-win-1.7.4-1.el6_7.2.noarch.rpm
  virtio-win-1.7.4-1.el6_6.noarch.rpm    (included in 6.7)
  virtio-win-1.7.2-2.el6.noarch.rpm      (included in 6.6)

• On KVM hosts running RHEL6.6, which version of virtio-win should we install into Windows guests on those hosts ?
  The latest one for RHEL6.6 (virtio-win-1.7.4-1.el6_6.noarch.rpm), the latest one for RHEL6 (virtio-win-1.7.4-1.el6_7.2.noarch.rpm), or the latest one RedHat provides(virtio-win-1.8.0-4.el7.noarch.rpm)
• When we move a guest from RHEL6.6 host to RHEL7.2 host, do we have to alternate the virtio-win driver installed in the guest by the one for RHEL7.2 ?
• Are there any combinations of RHEL's version and virtio-win's version which RedHat doesn't support ?