When asked if I give a discount for cash, I usually reply, "why would I discount for cash?"
To which they reply, "because you don't have to pay tax."
To which I question, "Are you asking me to lie to the government and my insurance company to save you money? I'm sorry, if I'm going to commit fraud, I'm going to keep all the money. What are you risking in this arrangement?"
Of course I only really said something like it once with a customer that could take that kind of a ribbing. Usually something like it can lead to kind of an awkward working relationship. I usually politely reply, "I'm sorry I don't." and if they continue to push I tell them I have plenty of tax deductions and will likely find a way to write it off anyway.