needle tank vent, open or closed during operation?

[TheBennyB]

Right on Giants, thx for the heads up. So is it a one way needle valve unless you use the press in ficture if wanted? To me if you have a 1//2 full tank you wouldn't want more air allowed into the tank and possibly cause bubbles or foam in your lines so you'd want it closed? Or does the rising and lowering (expansion/fluid in and out) of that 1/2 tank essentially keep it air tight? I've tried to read up on r/c hydro but the subtleties I'm still working on.