The Ideal workflow is to leave ALL the interpolation to QImage.
Start with your camera file in Photoshop, and do all your adjustments to colour, etc without resizing or sharpening. Flatten and save as a .tif file.
Bring it into the QImage queue at the size you want (make sure the Crop Button - the scissors - is clicked on), and crop it in the page editor. You can click the little magnifying glass to enter the precision crop area if necessary. It uses sticky crops, so the next time you load that photo at that size it will remember your crop. Adjust the sharpness level after you have seen a print.
This way the file only undergoes one interpolation, from camera to printer resolution, using the best algorithms in the business., which is why you get the best results.