I always pay for my car tyres in cash. Saves me about £15 per tyre. And I would happily avoid paying tax on other things if it was a worthy saving for me.
Depends on the job, whether the guy is VAT registered and how much its worth.
If I do a little job for a neighbour or friend it would be churlish to refuse a bottle or two. Somewhere there is a line between this and a new bathroom, which should go through the books.