When would I let the dvd player do the processing over the 4805?
You should let your dvd player do the processing over the 4805 if it uses a better deinterlacing solution over the 4805. Popular deinterlacing solutions comparable (and better) than the Faroudja processing used by the 4805 include Silicon Optix HQV chip (found in the Denon 5910) and Silicon Image SIL504 chip.