I'm asking in relation to my particular situation. All my images come into Photoshop from ACR as tagged images in sRGB, AdobeRGB, or ProPhoto RGB color spaces. I have my Photoshop color settings to always preserve the embeded profiles. (I do not have a check mark on Profile Mismatches: Ask When Opening.) If I understand correctly, Photoshop will ignore the working color space and use the color space of the tagged image, thus making the color of the image on my monitor appear correct. I can see where it would be important to visualize an untagged image in my chosen working space, but for tagged images, does it really matter what my chosen working color space is?