Hendrik was right, I was mixing bit rate and sample rate, hadn't had my coffee yet
For sample rates, the general rule used to be up sample evenly (I.e. 48-96, 44-88) but with the quality of filters these days I'm not sure it's a real issue anymore, with a high end converter like MC uses it's likely not audible
the majority of DAC's up sample (over sample) anyways, you should send everything at the "native" DAC rate. I have a DAC for example that is native 24/96 so I get MC to send everything at 24/96 no matter what