I have found that generally, if the temperature is below 70F, I don't need to worry about sweating too much in work clothes. As the temperature gets closer to 70 and above, wearing a backpack/bag is will allow sweat to build up under your shirt. Above 80 F, even without a bag over your shoulder, you will start to significant sweat buildup.

Luckily, it rarely ever gets above 70F where I live, in the morning, and the 100F commute home isn't a problem as I can change and shower easily once home. As such, I don't bother with cycling clothes, but I only have 5 mile commute.