Its your typical upgrade reality. If you are upgrading, and spending money on new parts, why risk destroying them because of wear and tear caused by misalignment? There really aren't many half measures in airsoft upgrades. You pull one string and the whole mess comes apart
I have purposely NOT shimmed the gearboxes on a few of my cheaper stock guns and they still work fine, surprisingly. But when they do go, I doubt I will be able to salvage anything and it would be a new mechbox or a complete gut and rebuild.