The manual on page 193 says, “whether to dither when mixing down to 24 bits is a matter of taste”.
I’ve also read elsewhere, " … only dither at the end of the chain". So, if it’s all going to a 16 bit CD, one wouldn’t dither first from 32 bit to 24 bit, and then from 24 bit to 16 bit.
Of course a single 24-bit file can be drug from the pool and converted to 32 bits in the project, changed in some fashion (volume, etc.), then brought back to 24 bits and replaced in the pool … multiple times. So I guess it’s a good thing dither isn’t being applied automatically by Cubase each time, right?
I guess maybe it’s all academic … any sonic degradation must be small enough to be unnoticeable.