You're right that the hanger can be straightened with the wheel on and clamped tightly, and that might solve the RD problem.

But there are other considerations. First the flexing puts the dropout out of plane and adds stress on the axle. Also, if the hanger sits at an angle, even if the derailleur mounting area is parallel it might be too far out or in, and beyond the RDs travel range for the cassette. Sanding it to establish better support and stability before squaring up the RD tab is a few minutes work and makes for a better job.