Part 23 (I only checked current regs though) only requires Cessna to determine the climb performance and confirm that it meets a climb gradient of 8.3 percent. If the C150 meets that gradient at a slightly higher (and thus safer) speed than the absolute Vx that they determined, they can certainly publish a different speed and having done so, you are then obliged to use that (at least when quoting it to an examiner). So use the figure from the POH that is valid for that particular C150.
If you ever have to get yourself out of a sticky situation (read: emergency) and using the lower (not from the POH) figure will prevent the accident, you'll be congratulated. If you ever smash up an aeroplane and they find out that it was (partly) because you flew a lower (not from the POH) Vx figure, you'll be blamed.