I'm not familiar with how 'tax men' work in the US, but in countries i'm familiar with government officials do not dictate file formats, and all communication is done in formats that are open standards. In the slim chance that some bureaucrat would continue to insist on using .xls or .xlsx, I'd probably ship him an exported file from LibreOffice, and say that it's the best I can do. And then again, being a proponent of Free Software and open standards, I'd probably refuse using a proprietary format. So far, I had some luck in avoiding using microsoft office file formats in the past 15 years of entrepreneurship.