I'd say it depends on your soil type and location. On my upland ridges, the water table is 8' - 12' below the surface. I've dug holes in April and returned later to find standing water. I didn't find water when I dug 32" holes on the same ridge during July. My new spring planted trees in this area look poor without being watered in late July - September. The established trees (planted previous fall or earlier) don't seem to need watering.
Of course, this example is my place in Carroll County. Watering trees in bottom land areas bring different considerations.
Blount Arrow could be correct, but "if" I planted this late in the season, I would plan on monitoring closely and being prepared to water during each visit.