The question is really about audio output modes on the PS3, not the audio formats on the disc. The audio track on the disc is either Linear PCM or a compressed format (which can be lossy, like DD/DTS, or lossless, like DD TrueHD or DTS-MA).
If you set the PS3 to output bitstream, then it will send exactly what's on the disc without modifying it at all, and it's up to your receiver to decode whatever needs decoding. If the PS3 is set to output PCM, then it will decode compressed formats into Linear PCM and send that LPCM stream to the receiver.
When you've selected a PCM soundtrack from the disc, then the PS3 will be sending PCM to the receiver regardless of the setting. However, as mentioned above, if the PS3 is set to bitstream then it won't be able to make any changes to the audio stream, such as mixing in menu sounds.
At this point, there's really no reason to select bitstream output on the PS3 unless your receiver has problems receiving multichannel PCM via HDMI. The PS3 is perfectly capable of decoding any of the available audio formats except DTS-MA (which is currently neither decoded nor passed as bitstream), and you get the ability for it to mix in additional audio.
The BD format is really designed for the player to do audio decoding anyway; the ability to pass the HD audio formats (DD TrueHD and DTS-MA) as bitstream to the receiver, which requires HDMI 1.3, is something that was demanded by the receiver manufacturers so that they could badge their receivers as "True-HD Capable" and "DTS-MA Capable". In reality, any receiver that can receive multichannel PCM over HDMI is capable of playing these formats, as long as the player does the decoding, which is what they're designed to do.