Locating terminal boxes above GWB ceilings is fine, and often must be done based on your (architectural) needs. It's just necessary for building maintenance staff to be able to access the damper control box, the coil cleaning access panel if provided, and the associated HW piping control valve, drains, and flush ports, if the boxes in question have reheat. This can often be done with a single, 18" x 18" access panel if properly located.
If you've heard that this is not acceptable practice, then you may have been mislead. Just make sure access is available, and the construction team should be able to demonstrate maintenance access if requested for building acceptance.
