As a rule of thumb, I tend to figure if I can't close the lid,the case is too tight.. <grin>.
Seriously, I think too loose is worse than too tight. of course you don't want it so tight where it damages the finish putting it in, or removing it. If it take any sort of force to push it into it, I would say that is too tight. I want it to lay in it, and be removed without having to apply force other then lifting it out. But again, I worry more in too loose. You don't want your instrument shifting around in the case because it can cause burns, or wear on the finish as it shifts around from being moved around, or travel. Also, damage to them can be caused not from external damage from bumps and drops, but damage from collisions inside of the case (the instrument slamming into the sides of the interior of the case) Your case could look fine from a drop, but ya open it up, and you have a cracked headstock, or body. So, I like them snug to limit the movement, but not so snug it take any force to pull it from the case.. Not sure if that helps, because of course like mentioned above, it is very specific with each instrument, and manufacture, style, etc.