It depends largely on the job. For example, we wanted 3 fence panels and 4 fence posts putting up a few months back, and the guy said £475 - or £315 in cash, and we'll come and do it next Saturday, because it's only a small job.
I'm willing to guess that taking it in cash, and saying I will 'do it on a Saturday' means that he will go and get the fence panels and posts for maybe £120 in total, and then pocket the other £200 ... and won't put it through the business/the books.
We went for the second option - cash in hand, and got the job done for one third less than what it would have been if you went through the books. Suits me ... It's up to him what he does with his money and how he dishes it out. It's not my problem if he's dodging tax.
If it was a big job though, like you know, multiple thousands, then definitely no. It would have to go through the books properly. Basically I would just be thinking about myself and my well-being first. I wouldn't risk a job that cost multiple thousands being paid in cash, but I would if it was a small-ish job (mid 3 figures,) and the price was lower.