I don't think you can file a chain "needlessly," since a chain begins to get dull the minute you start cutting with it. You're always ahead to cut with the sharpest chain possible. The chain will last longer and your saw (and your body) will see less wear and tear. It's just like the professional chefs or barbers who sharpen their knives or razors all the time. They always want the sharpest tool possible. As a practical matter, sharpening a saw chain when you stop for a break (or a refill) presents the best opportunity to touch up your chain, since you've stopped working for other reasons anyway. Fewer professional loggers use chain saws these days--at least most don't do most of their work with them--but those I visit who do use their saw full-time, always touch up their chains when they gas up. Obviously, if you hit a rock or otherwise bung up your chain mid-tank, you need to address the problem immediately, as trying to cut with a dull chain will not only further damage the chain, but it will prematurely wear more expensive components of your saw, such as the bar, vibration dampening system, etc.
This approach assumes that you know how to sharpen a saw, i.e., make it sharper when you're done than when you started. Admittedly, it took me a long time to get to that point, and I've tried just about every contraption and technique out there. Finally, I found that the best solution is a round file properly used and a flat file (sparingly) used on the rakers. I have heard good things about the relatively new Husqvarna/Pferd file holder that takes the rakers down while you file and although I haven't used it extensively, it seems to be superior to filing each component separately.