What is generally more "trustable" to visually correct the camera is to look at verticals rather than horizontals. Place a known vertical like the edge of a wall in the center of the frame. Be careful with wide angle lenses and having the camera tilted up or down--they will cause verticals on the edges of the frame to become diagonals. However anything in the center should stay visually vertical.

Bear in mind that quick releases and questionable tolerances in the camera mount may contribute to the camera being slightly off-whack from the head; leveling the head perfectly may not always result in a perfectly level camera. Best way to check this is with a simple tubular level; press it up against the underside of the camera base or on any truly flat surface on the camera and check it against the head's level. If there is enough of a difference to notice, considering permanently mounting a level on the camera itself.