Streaky,
O.k., not picking on you here, but you're talking about MAGAZINES, not "clips". (Semantics here

)
There are Clips, but they're sheet metal devices that by their own spring tension, or by their rim holding design, hold ammo in a stationary position until actuated by some external force, either the gun's or a finger or thumb, etc.
Magazines are multiple part "boxes" that typically have a spring and follower (and maybe removable baseplate) that move and FEED ammo to a certain position by their own internal mechanism.
But we know what you're talking about...
In any case, when USING, fill 'em up.
When STORING ammo in them, unload them, to half their capacity.
Obviously, UNLOADED is the best for the Spring.
Leaving fully loaded, all the time can allow the spring to take a "set" and they may not feed reliably after this happens.