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Wether or not you should do this depends on if you can jam your valve in 
the open position. In my shipyard days many valves needed to be slammed 
into the open position as this sealed the bottom of the packing gland 
against leakage. These type valves had stems which unscrewed from the 
valve body and it was real obvious as to their state. 

With gate valves with fixed stems if the valve had a state indicator (a 
small pointer which moved up and down a threaded portion of the fixed 
stem) then it meant that you were susposed to jam the valve in the open 
position to seal the bottom of the gland.

If there was no state indicator it was general procedure to open the 
valve all the way then back it up a bit. So, if you took a valve and 
turned it to the left and it stopped after a 1/2 or 1/4 turn, it was 
open. If it didn't turn, it was closed and you could then put the shit to 
it to open it. And yes, I did break the stem off of a valve which was 
already open one time. It took a 20" pipe wrench to do it. Remember, 
never force it, get a bigger pipe wrench.

As far as I know, you don't have to jam a HP seat valve in the open 
position to keep it from leaking due to the o-ring packing, so when fully 
open, back it off a wee bit to make it easy to determine it's state.

>Does anybody know the real story behind the practice still taught 
>by some agencies of turning tank valves a quarter-turn back after
>opening them all the way?  I've "heard" that at one point in time
>there may have been valves where this really was useful, but that 
>it has long since become an anachronism (and a potential point of
>confusion, albeit a small one, for someone who wants to quickly 
>ensure their valve is on all the way).  I don't turn mine back, 
>but on a couple of occasions some well-intentioned buddy or DM 
>has "checked" my air supply and turned it back before I explained 
>it (and in the case of the DM, just politely asked them to leave 
>me and my gear alone).  Obviously, I'd prefer to know the whole
>story before explaining to these guys why it's a stupid "rule".
>
>So what's the truth?  Was the quarter-turn back ever a good idea?
>
>- Todd

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