Yes you can cut them back after flowering stops. Cut the flag in an inverted V shape.

Does this actually benefit the iris, Scotsgran? We have a number of siberians and I never touch the leaves. However, I do notice a decline in the number of flower stems over a period of years. Would trimming the leaves help this or is it just to make the plant look tidier?

I honestly do not know. I do know that if bearded iris get surrounded by other flowers and the rhizomes do not get sun then there is a fall off in the quality. You can bury the rhizomes if the soil is sandy which yours may well be so you would only need to cut them down if they are blocking something else or are getting straggly. Iris sibirica is beardless but it still has rhizomes and mine is planted as above and treated the same way except that it is in a place that is shadier and therefore has more moisture at the foot although its head is in the sun. We will have to see what others say. I looked to see if I could find anything on this and copy here the description from

whiteflowerfarm.com
A very rich Pansy violet that appears nearly black. 'Caesar's Brother' is an old, award-winning variety and remains in great demand because few others can rival its deep coloring and exceptional vigor. A mature clump in its glory is simply breathtaking.
Iris breeders are dramatically changing the range of colors, heights, and bloom times available in this group. Siberian Iris often begin to flower in late May, but the peak of bloom here is in the first half of June. Their thick roots drive down deeply, which makes them drought resistant (though they also thrive in damp soil) and great plants for holding steep banks in place. Each clump becomes huge, and when a group of them grows together, weeds can't grow in between. After bloom, the grassy leaves stand erect and green through the growing season. Siberian Iris struggle in the desert Southwest.