Living in Massachusetts I've done various Autumn rides where the temp was in the 20's. If you're dressed to be warm on the descents you'll be roasting on the ensuing climb and if you're comfortable on the climb, you'll be freezing on the way down. I've generally opted to be comfortable on the climb and therefore cold on the descents. And there's nothing "flat" around here.
On the other end... Days in the 90's are pretty rare. I'd stay inside if the forecast was for a high of 95. Even for a high of 90 I'd be trying to start as early as possible.
I was doing a cycling tour in Utah one time and we had a century day heading towards Lake Powell... 100 miles across a desert with no shade? Ugh. I don't recall the temps, but that was one of the more brutal days I've ever had on a bike, temperature wise.