| but you can't just take it as a given that in-printer interpolation is
| worse than doing it yourself,

Are you sure that printers do resampling? I would
have guessed it depends on the software. For instance,
if I open an image in even a simple program like IrfanView
I can set the print size to whatever is desired. IV also
does resampling. Is there any reason to assume that
IV and other programs are not doing the resampling
themselves before sending it to the printer? I wonder
if the printer API even has options for resampling. That's
really not part of its job.