Not historically accurate at all. The film isn't exactly anachronistic so much at it is fantastic (as in, fantasy). House of Flying Daggers belong to a genre (wuxia aka Chinese martial arts films) that is built on intentional romantic anachronism.

Like most wuxia films, the movie House of Flying Daggers is historical in roughly the same sense that A Song of Fire and Ice is historical -- parts of it are loosely inspired by historical accounts of events that may or may not have actually happened. None of it should be taken as a representation of real events, or of daily life in the past.

Essentially the only relationship to reality is that the plot is loosely based on a poem by Tang Dynasty era poet Li Yannian (李延年).

House of Flying Daggers is set in a fantasy-version of medieval China, in much the same sense that Robin Hood: Prince of Thieves is set in a fantasy-version of medieval Europe. Skillfully done as an example of the art of the film director and choreographer, but treat it as fiction and entertainment.