trimming spent/diseased iris foliage at any time is beneficial from the standpoint of clean garden culture which pre-empts disease development and insect infestation.
HOWEVER ..........
Cutting the green healthy growth, just because traditional garden wisdom says so, is in my IMHO, an unproductive task. Please remember, that bearded iris, no matter where you live, never truly go in a bona fide "dormancy" in the summer.
They are in a modified growth phase, in which metabolites synthesized in the leaves are being directed down to the rhizome for storage until needed the following spring for growth and bloom. Also, anchor roots are being put down at this time. These anchor roots are gathering, primarily phosphorus and potassium, (as well as moisture). This P and K loading by the anchor roots is best combined with the sugars and other metabolites produced in the leaves for optimum starch, carbohydrate and sugar composition in the rhizome.
If you aren't transplanting or shipping iris, save yourself some work and hold off on the foliage cutting until fall, when the plants truly enter into winter dormancy
