I was running a Shimano cog at one point, and the chain blew off when I was cranking hard....needless to say, I went to the ground hard, and ever since I have run a tensioner for saftey reasons (its all in my head)...it works fine now with a proper SS cog, cause they have taller teeth and have more for the chain to hang onto....

 oh yeah, i actually had the EXACT same situation as you, and i am going to run a chain tug just in case...