-- AC3Filter is not needed? Yes/No? I wonder why I started using it...
It is not "needed" if you just want to play wave files that contain DTS (or DD) encoded PCM data - assuming your sound device (aka sound card) can output bit perfect data to SPDIF natively without resampling the output. If the manufacturer states that the device is ASIO compatible and provides an ASIO driver the device is more likely to be able to do that.
In any case you must first bypass the Windows mixer, which cannot be set to be fully bit perfect. Its mixer features are always available and like any high quality digital mixer it must add a very small amount of dithering noise in order to avoid audible distortion. This is desirable for standard audio playback, but it destroys any integrated DTS or DD code, which cannot be altered at all. ASIO (and WASAPI in the exclusive mode) can bypass the Windows mixer.
If the sound card has native support for the used sample rate and it can be set to output to SPDIF with its own control panel or if its SPDIF output is always active (like in my Terratec) it should be able to transmit unaltered audio signal.
if AC3Filter is installed and active and set to passthrough AC3 and DTS and output set to "As Is" will it interfere with bit-perfect?
It should be fine, but with 44.1 KHz files the sound card must support that sample rate natively. (AFAIK, all sound cards with SPDIF support 48 KHz without problems.)
-- why it is recommended to use AC3Filter/ffdshow to play multichannel sound?
It is necessary with video files and also with audio files when you want to decode the multichannel audio signal on PC and use an analog connection to the amp or powered speakers (i.e. connect the wires for 6 individual analog channels when the setup is 5.1.)
For DTS/DD wav audio files, SPDIF, and an external decoder/receiver the choice is yours. You may need to use the DirectShow playback system, SW decoding, resampling, and re-encoding if your HW does not support 44.1 KHz natively.