I do recall riding with no cotter nuts without thinking about it and never had a problem. This followed damage to the threads caused by unsuccessful removal technique. In thinking about it, I would say that riding without the nut is not unsafe. While it may pop out, it should give you some warning by way of a little play in the crank. When the crank is under pressure (stomping up hills) I'd guess that the torque would assure that the pin would not move.
Now that I am more prudent I would replace the cotter as soon as I could; however I would still ride it nutless if only in the hope that riding it would loosen the pin.