I've never done it as I was brought up in a very strict Christian family where any dishonesty was frowned upon.
I have a window cleaning friend, who does the opposite, always takes cash where possible and doesn't declare it.
The vast majority of my customers pay by bank transfer but I do still get people offering me cash, which I turn down. Also people trying to get a lower price by paying cash.  Had a lady the other day that did that and she was shocked I wouldn't take it. 
Just wondering whether I'm mugging myself off. Not exactly thrilled with how my tax is used by the government.
Do HMRC even investigate tiny one man businesses?
By the way I still wouldn't take cash even if I was going to declare it as I dont have a bank near me to deposit it with.
The post office down the road from us will take cash deposits into my bank account. If I present my debit card, the amount is credited to my account virtually instantly.
We still have a large number of customers who pay by cash. Every cash receipt is recorded on our software. Every day I balance my cash tin against the cash payment/s received. Cash tips are also included but listed separately.  (What I don't 'declare' is any bottles of wine, boxes of chocolates or boxes of biscuits we are given. How do we put a monetary value on these items?)
I've done a breakdown of our receipts over the past 2 months. 
45% of our receipts are cash.
54% are bank transfer
1% are cheques.
Cash tips included in the cash receipts come to 5%.
If I didn't declare my cash receipts or some of my cash receipts, then business running expenses would be skew and could show up on the tax man's algorithm. Firstly, running two sets of books is dishonest and secondly, I don't want to be looking over my shoulder every day.
Over the years we have found out that at least 3 of our customers have worked from the HMRC, and a number are also policemen.
The other thing is, imho, that it is not our concern what the government spends our tax bucks on.