Not tax evasion by the customer, who wants a better deal. Same goes on at stores and restaurants. We always pay in cash, many clerks say they have to pay 3% or more for credit card fees so they discount if you pay in cash. If a contractor gives a discount, who are we to blame him while he avoids nasty paperwork accounting.
As someone who has had too many unscrupulous contractors, who lied, did absolutely egregious work that I had to redo, and in one instance actually started my house on fire, I have zero desire to help them avoid paying taxes - even if it increases my price for the work.