First of all, you don't want to store a fully charged battery. It's OK to keep it charged overnight if you're flying the next day, but if you're not going to be flying for a few days, you should really store the battery at 50-60% capacity. Doing so prolongs the life of the battery (because a fully charged battery is under strain and will have a shorter life).
As to storing the battery in the Phantom, doing so will put pressure on the spring-loaded data pins as mentioned. It's not the end of the world if you do so, but if you can help it, it's probably better to store the battery outside the Phantom.