I have 0.28 and 0.38 and I probably choose based on the paper. If I choose one pen, it's the Uniball Signo DX 0.38. That seems enough for cheap paper and precise enough that everything is legible without extreme care.

If I were doing math I would prefer to use the 0.28 version and make sure my paper is good. Anything smaller and it feels like I'm drawing and not writing. I think the 0.2's are mostly used for art.