If you are talking about an internal mix with software, there is no conversion involved after the initial tracking. Assuming you don't need to use outboard gear during the mix or need to mix down to an external piece of equipment, the only conversion your audio will go through is the initial A/D (and D/A while monitoring of course). The quality of the sound card you are using during mix will only effect the sound of your mix to the extent that you may hear things differently through each set of converters and hence mix differently. The mixing itself is done in software and the sound card won't make a difference.