If your rubber stopper doesn't tighten fully, I would replace it. Now don't get me wrong, I'm not one of those guys who thinks "everything has to be perfect". Quite the contrary. But I HAVE had fuel leaks in the tank compartment, and it can RUIN your airplane.

And those rubber stoppers can be tricky too. In fact, MY fuel leak was caused due to OVER tightening a rubber stopper. It was so tight that it caused the seam in the tank to split.