I have seen way too many drivetrains in need of total replacement because the owner went way too far trying to save a few bucks on a chain. Bottom line is to change before it is worn out and use a good gauge (or ruler) often to determine when to change. Even if you did not get the last 300 miles out of the chain that you might have been able to squeeze, that is better than going too far. An ounce or prevention...
Gauges likely have an inherent error likely when in use (due to misuse or machining variance) so if they are a bit too conservative and overestimate wear that is better than the converse.