It depends on what's causing your bad run. If you're getting bad results because you're tilted and playing bad, going to play money isn't going to help. If it's variance that's causing your bad results, then going to play money isn't going to help either.
In my experience, when I'm running bad it's a combination of the two and it's better for me to just take some time off and not think about poker for a few days. I guess the moral of the story is that going to play money because you're in a downswing is a waste of time.