Do you always open your AEG gearbox when you get a new one?

Do you guys open up and tweak/check every replica you get or only ones you plan to perform upgrades on? I personally leave one completely stock as a backup to my modified or custom builds, and my recommendation to friends and family members is to leave their replicas unmodified until they break, but I've also seen some advice to check and reshim every gearbox no matter what. What are your thoughts?