Always connect the cords to grounded power outlets. Computer equipment requires electrical circuits to be grounded to Earth ground, and connecting the power cords to grounded outlets completes the server grounding. No additional cabinet grounding is required.
Because different grounding methods vary by locality, refer to documentation such as IEC documents for the correct grounding method. Ensure that the facility administrator or qualified electrical engineer verifies the grounding method for the building and performs the grounding work.