[EricH]

Originally Posted by MrBasse

I might be missing it, but we've never done this and have never been asked to do this. Care to cite a rule? I searched and came up empty...

There isn't one.

Now, as a matter of SAFETY, it may be determined either way as to whether it's safer to vent the air or to leave it in. I can think of one or two applications where it might be safer to vent, possibly, but stuff is more likely to stay put when there's air in the system.