It depends entirely on your environment. My yard is so well suited for animals that I'm not sure I'll ever have a heat problem. We only lost the one Ameracauna last summer and it was during our work day, so we aren't sure of the true circumstances. For me, it is miraculous what the grass yard does for the temperature in the back. Somewhere way back, I did a comparison between the grass in direct sunlight, grass in the shadows and the gravel yard itcs front. If AI remember correctly, it was 117* that day. The sunny grass was 98*, the shaded grass was 87* and the gravel was 125* or something like that...
Edited: See here for that discussion...
https://www.backyardchickens.com/t/31227/arizona-chickens/37770#post_11529428
My understanding is that summer litters will have longer ears, because that's the main cooling method for rabbits. Otherwise, just like any living creature, don't pamper them and allow them to adjust to nature. They have survived without our cooling methods for hundreds or thousands of years. They grow a different coat of fur for the season and everything. We tend to overthink things, rather than allowing Mother Nature to do her job.