joe-90 said:
...working the black economy - just join them. You can't beat them.
Nobody has to do either - the third option is to do what suits you and your circumstances. If you're an honest person, and/or someone who doesn't want the anxiety of wondering whether or not you'll be found out, then you'll do the right thing regardless of anyone else's behaviour.
For example, every penny that I receive goes through my accounts, whether it's a remittance, a credit for faulty component, gratutity, tip, etc. - you name it, it gets declared. Apart from a clear conscience, the main benefit is months worth of work stacked up without any advertising costs.
In answer to the original question, I detest any attempt to knock money off, and I don't tolerate it. Here's an example - last year I missed an appointment, through forgetfulness (and poor diary management), and when this happens (on small jobs) I have a policy of charging half of the labour rate - this recompenses the customer and hurts myself enough to prevent the same mistake in the future. So, the total bill was reduced to £47.50.
Here's the thing - he gave me £47.
00 in cash. I looked at him for a moment, silently, and thanked him for the money.
The next time he calls, I'll be too busy.