Yeah...everybody
should be charging sales tax.
As for working for "cash": If somebody pays me before I send a bill and just pays the cash price, I'll usually put it in my books as they payed me tax... For example if my service was $100, the billed amount should be $106 (assuming I was in a 6% tax county). If they send me $100, I'll record it as the service cost $94.34 plus 6% tax to bring the total charge to $100 with tax. I try to avoid this, but a few slip through.
It is not worth it to me to try to cheat the tax man for a few bucks. I also don't want clients to feel like I am trying to squeeze the last dollar out of them (this happens more often with friends-most standard clients pay me when they get the bill).
PS: no need to be offended at a question like that