I ALWAYS reply back - it's just common courtesy in my mind.

If a parent emails me asking for questions, even if it's irrelvant to what I'm asking for, I will email them back to say that I'm only available for ages 1-4, not 6 year olds (and let them know of another home childcare that's in the area).

In my business classes that I took last year, one of my profs always said, it's common courtesy to reply back, even if it's just to say thank you. In all, I always email back someone, because in time I might get another email from them in the future, or they have referred someone to me!