If cFIREsim accounted for taxes, it probably wouldn't exist. It would still be in the development phase. Can you imagine how much extra work it would be to incorporate every single possible tax scenario into the framework? And even then, how much more cumbersome it would be to use?
Like others said, taxes are an expense. They're an expense that varies greatly from person to person, so the best way to do it is to figure out your own personal tax amounts and count that as you would any other expense.