That's pretty much how it worked for my sister and me, too.

My girlfriend in high school had a soft curfew. If she was going to be out any later than 1 a.m., she had to call her parents. That was the only restriction she had.

I think this (lack of or "soft" curfew) encourages more responsibility and openness and minimizes potential resentment. It works far better than arbitrarily choosing a time.