If Bluetooth is enabled, but is NOT actually connected to a paired device, it will attempt to acquire a connection until it finds one, thus causing battery drain. Also, a weak cellular or Wi-Fi connection can cause battery drain issues.
I don't use Bluetooth, so I keep mine off. But is has been on at times. I've never noticed a change in battery usage with Bluetooth on. I suppose it uses a little bit. But many who use it leave it on all the time. I don't think having it on is a big battery issue. If you don't use it, turn it off. If you do use it, leave it on.