I always open my valve fully and then back about an eighth to quarter turn. This allows me to reach back and check the valve while it is on my back. If the valve is opened tightly, I just don't have the strength available while in the awkward position of reaching over my shoulder to check the valve. It just makes verifying that the valve is fully open easier for me if there is a little slack before the full open position.
While we are on the subject of having others check our valve before splashing, I have always gotten a 'buddy check' that included checking the valve. It doesn't bother me that someone checks my valve just before I splash - However - Last year, on a dive boat in Cozumel just after the group splashed, we discovered that three of us had valves that were closed - I know that I didn't close them. Guess it is easy for a DM to get a little confused sometimes.
Once on a Cenotes dive, I was paired with a couple who claimed that they were both instructors. As we walked down to the entrance to the Cenote, I asked her to check my valve (because I didn't think that I had turned it on). After a short wait, she told me that something was wrong and that the valve handle had come off the tank. She then proceeded to hand me my pressure transmitter that she had unscrewed from the regulator. Oh well!