Yes, romanians do make this distinction and it is the polite thing to do. If the woman you are talking seems unmarried (as in there are no obvious reasons for you to think she is married), 'domnișoara' is appropriate. Usually, when it comes to young women, we don't ponder that much on the married-not married aspect, unless we have strong reasons to believe they are married. And I'm saying that because somehow, 'domnișoara' is associated with young women. As a fact: there are 50ish women who proudly introduce themselves as 'domnișoara X', so don't worry about using this word :)