I wouldn't. The condensation that creates the fog effect could ruin your parts. It would probably be fine for a little bit, but I would not risk it. If you are really set on testing the airflow this way you might be better off using smoke. To be honest though, so long as you follow basic airflow designs you should be fine. Usually bottom/front are in and back is out. You want to make sure you have more air coming into your case than leaving, to maintain constant pressure. So usually that means more fans bringing air in and fewer expelling it. Of course that all depends on the fans you are using.
Edit: Tho I would still be careful with smoke, as it is conductive as well. But you will be fine if you don't use much.