The philosophy I use with any sort of DB movement is how far down would I be able to take the weight if I was using a bar? So if I am doing DB bench press I basically go all the way down to my chest but not any further and carry this into exercises like shoulder presses and flys. I think on some exercises you could get away with going deeper but I am not sure what the benefit would be... all I see is putting needless stress on your body.

That being said I also suffer from shoulder impingements so I am much more cautious now.