It'll probably be fine. I rode for a year with a stamped cog that had been rotafixed onto an old freewheel hub, with no lockring. That wheel and cog is still in use by a friend of mine on his junker conversion.

As a matter of principle, I wouldn't want to use a cheap cog on a good hub, but if its a cheapo beater, use whatever you got. There's no sense wasting money if you don't need to.