Scott Ferris , Nov 07, 2011; 11:12 p.m.
This is a can of worms :-)
Generally, if I am within 10-15% of where I need to be, I will Scale To Fit, this just works in the printers driver and takes the size adjustment needed in with the actual business of rendering the image from the file it is given to the actions of the print head. Printers are superb at scaling, mainly because they don't "see" pixels, they are interested in shading and gradations and work in much much smaller chunks of information, it takes thousands of ink drops to make one pixel.
If I am outside the 10-15% I will resample with an appropriate method and PPI. I do this because the printer needs x amount of information per inch, after all it doesn't know if I am trying to print a chessboard or a grey scale ramp, it needs enough pixel information to determine that first. If you push Scale To Fit too far you get pixelation.