1996 Buick Regal in the morning when engine is cold has rough idle after it warms up it idles smooth.

The main thing that comes to mind is that obviously when the car is cold the exhaust temperature is cold and then the exhaust warms up. The only thing looking at the exhaust is the O2 sensor. These are usually fairly inexpensive and easy to replace.

Does the car smell rich when it first starts up?

I'm not sure the IAC valve would be the culprit since usually if they're not working right you'll have rough idle at all times, but it doesn't hurt to remove it from the throttle body and clean the pintle and seat.