Harmonizing in 5th's is playing the fifth note in a scale above a given tone. So if you had a melody in C major, and you played B. You would play five notes (letters) above B. B,C,D,E,F. You would play F, not F#. Harmonizing in fifths doesn't mean play a power chord or a perfect fifth. You could play F# but that would have a stronger chance of sounding wrong since F# is not in the key of C major.