I agree, if it ain't broke, don't fix it. Check if it works before fiddling with anything. The one time I'd disagree is if there is a well known issue with the printer that is fixed by updating firmware, or if it's just to enable some safety features cheap kits often deliberately disable. Min-temp and other thermal runaway protections are the usual suspects.

Other than that, the only good reason to update firmware on a working machine is if you're making an upgrade that needs updating the firmware, like adding a leveling sensor or the like.