From his biography ("The Snowball"): "He did the bookkeeping himself and prepared his own tax returns. With its numbers, accuracy, and the measuring of results

From his biography ("The Snowball"): "He did the bookkeeping himself and prepared his own tax returns. With its numbers, accuracy, and the measuring of results, the recordkeeping aspect of the job pleased him." [...] "Warren was still handling all the money and doing all the clerical work himself: filing the tax returns,..." [...] "'Bill started trying to convince me to get a computer. I said, I don’t know what it’s going to do for me. I don’t care how my stock portfolio is doing every five minutes. And I can do my income taxes in my head.'" Seems like indeed he is doing them himself (his personal tax bill; to do it for Berkshire, they surely have hired help)