Since I started window cleaning (about a hundred years ago), I have always had a float of £25 in change on me. I very rarely have any trouble with changing notes, except one customer, who is always the first on that day, will give me a twenty pound note for an £8 job. I see it as part of good (and efficient) service. Having said that, some people do say to keep the change. I had one the other day, £12.50, he was out cleaning the top windows of his conservatory because had had done the roof, he said I didn't need to do the top windows and gave me £15 and said keep the change..