If the cutters are covered with crap, usually they are dull. That's because the unwanted friction heats the cutters and some debris melt/dry and stick on the metal. A sharp cutter stays clean most of the time. The dull cutter feel smooth under the finger because its edge is rounded. It catches the light and looks shiny.
By filing, you want to suppress the rounded part to come back to the straight adjacent surface. But you need to remove all of it, or the edge just slides on the fibers instead of biting them. Problem, the finger can feel an acute edge and be happy with that, even if there is still a bit of worn area. So, the finger isn't a good test. It's more accurate to try to catch a light reflect on the edge. Move around the bar to vary the angle from the light source. You see something, a tiny bright spot, file more. You can see nothing, that's good, go to the next one.
The saying "that will be good enough" doesn't work too well here.
Of course, if you have to wear prescription glasses, do it.
Be sure to use a sharp file, otherwise the burr may give you a wrong indication (plus that doesn't cut well).
If the chain hit some metal or a stone, the damages to the cutters can be massive. The edges are abraded, crushed, bent...
If they slide on a stone embedded in the stump during a few seconds, the cutter's top is heavily abraded and the loss of metal can extend way back behind the edge. If you want to save that chain, you need to remove all that damaged area. That could be so much as 1/8" to file on each tooth. A lot of file's strokes.