In Photoshop, I've been taught not to carry out adustments from the Image menu but to do each adjustment in a separate layer and flatten at the end, as this is less destructive of the image. I normally do minimal adjustments in ACR (3.3), mainly shadows and highlights, maybe white balance. Does this also impair the image?
No, the adjustments are only applied (rendered) when the RAW file is saved out as a TIFF or exported to Photoshop. The adjustments may be thought of as a set of 'instructions; these are stored with the image file but do not affect the original data.