The Mimi are the "ears" of the Japanese plane blade, the facets either side of the cutting edge. Their purpose is to allow the cutting edge to present itself to the work and take a shaving, without having that shaving jam between the blade itself and the channels of the block that hold it in place.
When assessing the mimi of your plane, the question to ask yourself is, "Is my blade's cutting edge wider than my block's mouth opening?" By mouth opening here, we are measuring it between the innermost edges of the channels that hold the blade.
What if I Don't Tune the Mimi?
Having a cutting edge that is wider than the mouth opening means that the edges of your shaving often end up crammed between the flat back of the blade and the channel that holds the blade.
Once a portion of a shaving becomes stuck, the shaving arriving behind it faces an even more significant dilemma with even less room to escape. Getting clean cuts becomes difficult, and if shaving detritus protrudes out of the mouth, the plane may stop cutting all together.
Thankfully, the mimi is a very simple area to tune. The aim is to effectively shorten the blade's cutting edge from either side - some planes require tuning on both sides, some on one, and some on none. It can be done with the blade in a high state of tune or before any other work has been done on it.
It is important, however, to know where the blade will bed into the block before starting. By bringing the cutting edge corners only just inside the mouth opening, the maximum possible cutting width is preserved.
We generally just work the final couple of millimetres of the mimi closest to the cutting edge. This is a quick process on a thousand grit stone, as there is so little metal to move.
And Once It's Done?
With the cutting edge sitting inside (but only just inside, mind) the kanna mouth, you have removed a possible source of snags and irritation.
Bear in mind that as you sharpen your kanna over time, the cutting edge will grow as material is sharpened off the bevel. Therefore, the mimi will shrink and after some use this area may need to be touched up. How quickly that occurs will depend on the angle used to touch up the mimi during these initial steps.