Its not always needed to release the pressure. However it is a good practice. Anybody that has tried fuel the tank with pressure in it can safely say that fuel will squirt about 5-10feet from the pressure inside the tank.

Another reason is if you decide to do work on the engine and flip it upside down....then you can't release the pressure as the vent line is not submerged and it will squirt fuel a long ways from the pressure.

Releasing the pressure after each flight will be nice to your fuel tank. Some tanks are not really good at holding the pressure and will get stress cracks over time. When you release the pressure you are helping this situation. If the tnak is under pressure and that pressure causes a leak...you could walk up to a model that has a fuse full of fuel....

My advice is always release the pressure in the tank after the flight. It won't hurt the engine but it could cause damage to the tank over time.

Troy Newman
Team YS