When we were taught lab practices around gas cylinders, we were told open it all the way then back it off like 1/4 turn. You would not believe the number of times people were wailing on an already open valve trying to "open" the obviously "stuck" valve as it was hard up against the full open stop (and occasionally damaging one). This way, either way you go to move it, you know it is free and open. There were no concerns stated about sealing only in full open, and we could be using some pretty toxic stuff at times, so if they had to be fully open to seal the valve stem, they would have said that...