I vote with the heat index folks. Humidity and bad air quality multiply heat, so 102 out west or the mediterranean where is dryer (if no smog) may be fine but here, in the Mid-Atlantic, there are times at 92-95 with humidity you just feel you're going to melt.

For me the other factor, whether we're talking hot or cold, is WHERE I ride. If it is cold or hot, I tend to do shorter and hillier rides. Our hills are not many miles up, but you build heat on the uphill and get a breath of fresh air on the cool slide back down. Flatter just leaves me slightly chilled in the winter or moderately hot all the time in the summer, since there is no resting time.

God bless those wicking fabrics! I tend to wear some sleeve even if it is hotter than blazes for the wicking effect, rather than let my skin just drip....

That being said, you need to know how to hydrate for yourself, your weather and your ride without taking in too little or too much. I know where every farm with a hose available to cyclists is and often pick my routes accordingly, carrying drink powder to mix instant refreshment.

Go for it!