The taxi ride will likely be longer than the walk, and be quite convoluted. If you walk up Powell St. and then turn on O'Farrell, you will be fine. Even if you are a few blocks west on O'Farrell, the worst you will encounter is a few hookers and an offer or two for non-alcoholic mind altering substances. (Note, this would not be the same advice for streets just south of O'Farrell, west of Union Square.