Gentlemen,
as far as I know, the first goal of a TOGISHI is to sharpen a blade, and if I remember correctly to have read in a book, very early blades were sharp but had no 'artistic' polish in a way to show HADA and HAMON. In later times, the TOGI techniques were refined but mainly on high quality blades. ASHIGARU often carried sharp but simple blades without any artistic merits.
In Japanese culture the care and maintenance of cutting tools and blades is obvious. Still today, housewives are able to keep their knives sharp with traditional water-stones, and craftsmen did (and do) this of course, too.
So it is not too far-fetched to assume that a soldier could sharpen his blades by himself, but I cannot believe that this was done on a wall, especially as walls were not built in a way to produce a very even surface. The only exception in doing this was probably in case of a damage like a broken tip. I do not believe that sharpening a blade or a YARI was done prior to a fight on a regular basis.