The modern solid core golf ball will firm up as they sit, though compression should level off after a while. If your inventory is just a few years old, you’re probably fine. Balata is a different story. Over time (and it doesn’t take much time), the liquid inside the ball evaporates. As George Constanza would say, there’s shrinkage. Any leftover inventory is going to be significantly lighter and smaller than it was designed to be. Incidentally, shrinkage is the reason why direct comparisons between modern balls and left-over balata inventory are basically invalid.