It's difficult to reach my tank valve when practicing, let alone at depth in an emergency. To reach back, find it, grasp it and turn it a dozen or so times is, at least to me, a herculean task which I would not realistically attempt since there are better options for me.
Those include a) carrying a second fully redundant gas source in the form of a pony bottle with valve open, transmitter and wrist mounted receiver for constant readout of tank pressure, and b) planned strategy of removing my rig should I need to turn off my main tank valve or address another issue with my gear that I cannot easily handle while wearing my rig.
I periodically remove my rig at depth for practice. I use the overhead technique to don it, rather than reaching through the shoulder straps which can be problematic.