It really depends on your climate. If you're in the middle of the desert and it's 100F when you're riding into work...might be **** out of luck.

Realistically though...for most morning commute times it's rare that it will be so warm you'll be a sweaty mess. I used to have a 12 mile each way commute here. In Chicago...it's pretty hot and humid in the summer. But at 8am...it's pretty rare that it's hot enough you'll just be a sweaty mess if you take it easy. That said...I DID keep spare clothes at work, and rode in in cycling gear, then changed once I got to work. 10 miles in the summer in dress clothes is not a good plan...