I know this is always an issue that people have varied opinions on. Here is my experience over the last 16 years. And this is based on the fact that I'm almost at the level of obsessive compulsive about getting every cutter perfect and all of them exactly equal, as in measure with a micrometer within a couple thou equal. Almost everything I have is Stihl RS.
Based on reading on here years ago the opinions that filing is the only way to really get the cutters sharp and keep from burning them, I tried hand filing and about every kind of tool or guide I could find, granberg, timberline, 2 in 1, etc. I could never get the results I wanted without spending a whole bunch of time on it, to the point where it got to be such a chore that I'd often just go buy a new chain or two to keep up with the cutting I needed to do. The timberline was about the best I found but still slow. Yes, nothing puts the sharp on a chain like a file but...
After 10 years, I finally broke down and decided to try a grinder. I think the Oregon 520-120 is what I have. The pink wheels that it came with I found to be very difficult and slow to use without getting the cutters hot, so I bought CBN wheels (1 for 3/8 and 1 for 3/8 lopro) almost immediately. At first I noticed too much lateral and radial runout for my taste, several thou of lateral and maybe a thou or two radial. It was enough to really notice when grinding and enough to set up some vibration. This was not the wheel. I trued up the arbor to fix the lateral runout and carefully put a piece of thin tape on the inside edge of the arbor hole of the wheel so that it fit more snug to the arbor for the radial runout. That seemed to get it within a thou on the runout both ways. Based on what I've read others have written I think this is not that uncommon to have the arbor not be true. Not a problem with the pink wheels, after a bit of dressing the edge winds up true but the CBN wheel does not wear so the arbor must be fixed.
I also read about reversible motors. I do need to rewire my grinder to be able to start backwards for the left side teeth but what I've done for now is just spin it backward with a drill with a piece of allen wrench chucked in and into the arbor nut. Once spinning at full speed with the drill, the grinder can be turned on and it will start spinning backwards to be used for the left cutters. There are varied opinions about this too, some people swear it doesn't matter but it's easy and I feel like grinding inward on both sides eliminates a chance for things to be unequal. I was always taught to grind away from any edge, not toward it, because it pulls the heat away from the edge. I'm sure a machinist would be able to tell me if that's true or not.
Once I did that I find I can get the teeth all perfectly sharp and equal without overheating the steel by just using very light feed pressure and being patient. I still do the rakers with gauge and a flat file on an extra bar in a vice. Rakers are fast and easy. The result is a chain that's perfect every time, usually requires very little material removal because the angles are exactly the same every time, and I dare say the edge is comparable to what I could get with a file which is to say razor sharp. They cut at least as good as new, smooth, straight and fast. They stay sharp for at least a couple of tanks in clean wood so I know I'm not screwing up the temper. And it's not a massive time consuming job to grind a few chains and get perfect results every time. So, grinder for me even though yes, I have gotten incredible results with files. With wheels it was north of $500 but has been paid for with my time and chain life I've saved being able to just take a couple thou off every time.
