Better to have it and not need it then need it and not have it.
I will be putting them in my box, this year. Plus having a white box helps reflect heat.
Put them in can't hurt especially if you have a wet dog in hot temps.
My box didn't have one when I got it. I made it about 2 weeks into the first Texas summer before I decided I had to add one. My box is fiberglass laid over foam insulation board so it stays a lot cooler than most, but not cool enough I would have been comfortable in it. I figure if it wouldn't have been comfortable for me it certainly wouldn't be comfortable with a dog that has two coats...