I don't think that's true.
EDIT: ok, maybe my immigration lawyer was a weirdo :)
The actual wage: This is the wage paid to other employees in the company who do the same work.
The prevailing wage: This is the wage for that occupation in the geographical area.
See: https://en.wikipedia.org/wiki/Labor_Condition_Application##1... https://en.wikipedia.org/wiki/H-1B_visa#Labor_Condition_Appl...
No, this is only part of employment-based greencard (LC) process. For H-1Bs, as part of the LCA process, the employer needs to advertise that a candidate is going to be hired.