I close mine to do the negative and positve test but I always have a bit of a battle to ensure that it is clicked securely into the unit. I always have mine open on 5 clicks while I dive the only time I ever notice it playing a role is as I ascend and I tend to do a few hard breaths out at various intervals as I ascend to get the OPV to kick in and reduce the loop volume.
I know my other half dives with it closed all the time and he prefers it that way.
I know that we also never roll in backwards wearing the units even if it means an undignified climb over the gunwales. Not quite sure why - I just do as I am told
Sally