Currently QPainter always paint fully transparent pixels as transparent black (0x00000000). QPainter with compositon mode CompositionMode_Source should just copy source pixels to destination without any blending. But currently, when source is drawing to transparent destination, pixels with zero alpha are multiplies to zero.
While color channel for transparent pixels is usually not important, there are some valid cases (e.g. scaling/antialiasing when using the resulting image as a OpenGL\Direct3D texture) when saving transparent background with a specific color is critical.