I have a check valve in one wingtip for an entirely different reason
than discussed thus far.  Once I started flying IFR I became concerned
that one day an accidental encounter with ice could plug the fuel vents.
 Mine are 3/16 tubing sticking about 3/4 inches below the wingtips with
the ends bent forward 45 degrees.  In the right wing tip, the vent line
has check valve teed in.  If the vent becomes plugged, the tank can vent
from inside the wing tip.  Under normal operation a slight positive
pressure is maintained with the check valve closed.
On a related note:  Early on I noticed that if the sun was just right,
I could see the rib location out on the wing.  It looked like the vents
were pressurizing the tanks a bit more than intended.  I connected a
spare airspeed indicator to the vent line and static line.  Sure enough,
the wing tank was subjected to nearly 100% ram pressure-both airspeed
indicators were showing nearly identical speeds.  I filed off the bottom
of the vent tubes until the pressure was reduced to 0.15 psi in cruise.
The skin no longer  bulges and you can't tell where the ribs are.
Chris Zavatson
N91CZ
360std