I really disliked tenderloin when I lived in SFO, its like its own little world people don't seem to walk out of (its not very big at all). The criminality there is much higher than anywhere else in the city; rents are also much cheaper (but that's only because elsewhere the rent is insane, there its still expensive)
In North Beach, Telegraph Hill, Financial District, Russian Hill, Union Square, Nob Hill, Fisherman's Warf and and SOMA, I didn't see many, if at all.
I lived on Telegraph Hill, next to north beach and even if the red light district was right down the hill on Broadway, I don't recall seeing TS's there; maybe I was blind!