Picture of a High Voltage Direct Curent powerline (HVDC line) on the Læsø Island, Denmark (from Google Street View):
We can observe that there are 2 conductor cables, and that both are insulated from the tower (which is grounded). Because the line is in DC it makes sense to have 2 cables.
However, I see no reason why one of those conductors would not be grounded. For example on railway electrification, one of the conductors (the overhead line or 3rd rail) is always isolated and the other of the conductors is the rail track itself and is grounded.
Grounding one of the conductors on the HVDC line might save money on isolators, not to mention electric and safety issues if the HVDC line as a whole recieves a too large voltage offset compared to ground.