I always keep my volume level at least one level below what I feel is normal. It never hurts to lower your volume level and considering all of us audiophiles are spending hours each and every day using headphones we should all be extra mindful of the harm of loud , continual sound can cause.
The HD650 is one of those very rare headphones that doesn't suffer from low volume the way that many "in your face" headphones do. I can have my HD650 at a very low , quiet level and the detail is still fantastic. Of course a high quality amp and dac will significantly effect this to a degree but I just think the HD650 excels for low volume listening.
The only time I turn up my volume is for certain games and never for longer than an hour.