Whether or not you agree, the maker of W7 says it is unsafe. You are likely beholden to keep data as safe as reasonably possible, and possibly to an even higher standard.
Defense is expensive, and using a known unsafe OS is impossible to defend, even if it is just one machine connected to your network. You would not likely find an arbiter, judge, or jury which could be convinced W7 is safe. Much cheaper to stay current.
I doubt you will find any tax software vendor which will give you explicit instructions to use a known unsafe setup. If you do, document their instructions, and pass it by your E&O insurer for approval.