We don't know for sure.
On my 920 though, I have not noticed a difference. If you turn it on and still are happy with the battery life, go for it. I believe what little power cost is well worth it.
I tested this out on my Lumia 521. Usually, when its off, I come back from school with 90%. With it on, it was at 80%, so it was a big difference for me
I tested this out on my Lumia 521. Usually, when its off, I come back from school with 90%. With it on, it was at 80%, so it was a big difference for me