A seal that is too loose may not be able to "keep" a seal which can inherently let it break (bad). A seal that is too tight may not be able to adapt to the ever changing dynamic of the ear canal through jaw movements, head movements, etc. In such a case, it too will lose a seal for being too stiff (also bad). What you need is something in the middle. Generally speaking, with the way customs are manufactured, the second instance becomes very difficult to achieve. Regardless once you obtain that proper seal, how "tight" it is won't matter unless it causes you to lose that seal.