In business classes back in the day, I was always told to simply charge a little more, and included "Consultation fee: --WAIVED--" on invoices.
Taxes in my province, regarding the graphic design profession at the time, were not at all clear-cut, though it was understood that graphic design services were non taxable (but some did get burned). Since consultant firms' services
were taxable though, many GDs in the province made a practice of stating outright that they weren't charging for those kinds of services. It also had the added benefit of looking like they were doing the client a favour. Any little trick in the book...
We have a harmonized tax now though, which pretty much means if you give someone a piece of
paper, you tax it, so I don't bother with the "waived" thing anymore.