If you are given bearded iris rhizomes in June there is no advantage in waiting to plant unless you're having 100 degree heat wave and may have to be rushed to the hospital.

In cold winter zones plant at least 6 weeks before the first frost so the beardeds will have time to put down roots.

Growers are busy dividing after bloom finishes, and then shipping. Bloom is done here by end of May and perhaps end of June in your zone. Hence the July/Aug availability of rhizomes.

Beardless iris another story.