Well, the first thing to remember is that MP3 is a lossy compressed format. I'd only ever convert to that immediately before delivery if you have to use that format. It has no place in the production process.
This means that you should mix down and "render" (render is a description in certain brands of software--it's not a phrase used in all DAWs) to an uncompressed format, i.e WAVE or PCM. (PCM is a generic file type of which WAVE is a subset--but by far the most commonly used subset).
As for the choice of 24 bit, that's also dependent on the software you use. Some are still locked at 24 bit integer for their "quality" work but many have now moved to 32 bit floating point operation and, where available, this is the way to go, again at the native sample rate.
Once you're finished your production, this is the time to convert to 16 bit 44.1 stereo wave format for a "glass master" for CD burning and (cough, gag) MP3 if you must. There's a whole other debate about the best way to achieve the conversion of bit depth and sample rate--to dither or not to dither and so on!
However, MP3 should only ever be a delivery format--it has no place whatsoever in the production or mastering process.
Bob