I had something like 30¢ of interest income from one bank this year. I entered it on the free fillable form for my federal taxes, but it rounded it to zero.

I looked through the instructions on the forms, and they say you may round to the nearest dollar. In theory, I think you'd need to compute capital gains (sell price - purchase price) at each transfer, but I expect in practice they'd almost all round to zero since USDC is supposed to be equal to the value of the US dollar.