You can drive with a bad IAC Valve but the bad IAC valve can cause engine performance issues and even cause the engine to stall on you.
If your IAC valve is going bad or is bad you really should get it replaced as soon as possible to prevent a break down.
Your fuel mileage will also suffer as a result of the bad IAC valve not controlling the vehicles computer properly when then can send too much or too little fuel into the engine.
So your engine will lose power and have acceleration issues, Irregular idle speed and your check engine light may also come on.
Irregular idle speed is one of the most common symptoms of A bad IAC valve.
I have driven with a bad IAC valve before when I couldn't afford to replace it but I would recommend replacing the IAC valve as soon as possible if you can afford to do so.