I do not agree with the "never sing the song from the show" idea.
first of all, if you do sing a song from the show for the character that you know you are right for, and can perform readily, and have done your homework on it, and found the character, then that shows the producers and the director A) that you know how to prepare for a role, and know what preperation implies.
B)that you show genuine interest in that specific role, and have made that commitment to the part ahead of time. this also shows the director that, if you are good, he won't have to worry as much about your ability to play the role, which is a great relief for directors.
C.) the director will not have to take that extra step in trying to imagine you singing fontine's song when you come in singing Fontine's song, belt it out, and give him everything he needs in one audition with any extra thought.
D.) it also shows that you are are at least familiar with the score, other than someone coming in and singing something completely inappropriate.
a director sees hundreds of people a day for a role, and it seems to me that he will remember you more if you sing the song of the character you are auditioning for, becuase he is already in that mindframe of looking for just the right person for the role. If you don't get it, too bad. that day they weren't looking for you, but if you are prepared everytime with the appropriate song from the show, eventually you will have what someone is looking for.
I don't think it is tacky at all to show that you have cared enough about the audition to get well educated with the material ahead of time.
Now obviously, if it is something like Wicked, if you are blonde, don't sing the wizard and I, and if you are a brunette, don't sing popular. know what character you are best for, choose a song that he or she sings, and then wear it out for the casting director.
"I'm an American, Damnit!!! And if it's three things I don't believe in, it's quitting and math."