A DVD player's 5.1 direct output uses the player's onboard DAC's and surround decoder - it's not something I'd recommend in general (exceptions do exist, but they're going to be rare) specifically because receivers generally offer more robust decoding (options like Pro Logic IIx, Dolby Digital EX, and DTS-ES), better bass management, and in many cases better DAC's.

The optical or coaxial digital output of a DVD player should not pass through any decoding within the DVD player - the output should be set to "bitstream" in the player's menu so that the data is passed directly from disc to digital audio output. Now, if the player's output is set to "PCM" instead of "bitstream" then the player has to decode the audio to create a stereo PCM signal. Since that discards all surround data (Dolby Digital 5.1 becomes PCM stereo, and DTS 5.1 typically doesn't output at all because many players lack DTS decoders), you should always select "bitstream" rather than "PCM" if you have a surround receiver or processor in place.