I think that some people get caught up in the "vanilla sound", and just get turned off when something sounds alittle dissonant. Dissonance is an important part of music because it creates interest, without it the music will sound monotonal. Of course every note sounds right, but sometimes right isn't right and having something alterative is better. It gives the music melodic character to add imperfections, in the same way people's imperfections give them character.

If the lead part is a good melody, and the harmony compliments the melody. Regardless of the notes used it's right. Non-harmonic tones usually resolve to a chordal tone, whether there a passing tone or on a beat. So F# over Am will commonly resolve to E or G# as a connection tone to A, or less commonly down to C. If it goes F# - C - the next note is usually C#. There's three different ways to use F# over an Am chord in A minor. There are others, but those are the most commonly and easily acceptable by the ear.