Seems like a lot of discussion on hand filing. Knowing how to hand file should be mandatory for anyone running a chain saw. But being able to keep all the angles proper and the rakers at the right depth is a difficult feat to master.
As I think I've posted elsewhere here I'm into chain saw milling.
I generally need to either swap or "touch up" chains about every 10' to 15' of 20" hardwood to keep every thing running smoothly. So my process is to hand file while outside, usually with a file and a Carlton File-O-Plate (FOP). That's the $5 solution.
Then when I get them inside (generally after having been "touched up" three times) I use a Granberg File'n Joint (a fancy file guide/jig) which is the $30 solution. This is probably good enough for most firewood cutters, especially if they don't rock the chain.
Then after 3 or so "Granberg" sharpenings (which would include about 9 or 10 FOP filings) I put it on the NT grinder to "true" all the angles up. That's the $100 solution.
One of the problems if you get your angles off is that the cut starts to curve a little, not much of a problem on a 2' crosscut, but a major problem on a 10' rip. Another is if you hit a nail or a rock and damage a few teeth, it's easier to grind them to get back in shape.