The Yasurime (file-mark patterns) are an important consideration in kantei (appraisal). The condition of the surface of the nakago (i.e. the depth of patina) gives an indication of the age of the blade, and the yasurime can, in conjuction with information gained from other aspects of the blade, give valuable pointers to the maker and school.
The nakago of a Japanese blade is NEVER CLEANED under any circumstances. To do so would seriously reduce its value.