I'll try and get the point across one more time. Wavy horizons are caused by wrong leveling in the stitching software. Even when the camera wasn't perfectly level, the wavy stitching can be corrected by choosing an appropriate horizon (mostly pitch parameter, most likely together with some roll due to sloppy technique) when stitching.
Having said that, there is a benefit to do a proper leveling before shooting. It wil allow to crop less of the info from the final stitch, or reduce the need to (content aware) fill in areas that were not covered by our image tiles.
Cheers,
Bart