Steve N wrote:Ah so it's good to make a custom frame job look like it's been framed in a standard size frame

No, but I get people wanting things mounted to fit a frame of the wrong size and the wrong proportions.
It's never easy making everything fit properly. I give a discount for those willing to use up my left over mountboard scraps. I recognise some of these frames as old frames that customers don't want back, after having an item re-framed and I give these frames to the local charity shops. Up-cycling is a popular thing around here and some people are framing stuff to sell on various internet sites. One of these sites is Etsy, but there are others too.
Many of these are non-standard sizes, so they can't get an off the shelf mount from the discount shops. I can't say that I make a fortune out of cutting a mount, or two every now and then, but it's easy money and some of these customers keep coming back, for more mounts. We have one lady, who is a well known artist and specialises in framing her work in old frames, mostly old Victorian and Edwardian. Over the years we've cut lots of mounts for her.