Me and DH had this conversation. I thought that he would say he definitely would (very moral), but he said the same as the two previous posters. depended on crime, age of DD etc. Whatever, we decided it would extremely hard to do what was right.
Twice my son stole from his Primary school, both times the school and I delt with it. School with a days exclusion, Me with holding all privledges and grounding him. If there is ever a third time, I have told him I will not hesitate to call the police.
I think it would very much depend on the circumstances - what law, why they broke it, whether it was a one-off and they realise their error or are planning to keep doing it, whether I think it's causing harm to them or others.
If DS smoked a joint, I'd explain why I thought it was a bad idea, but wouldn't report him to the police. If he started taking heroin I'd think about whether the police getting involved would be the best way to help him or reduce the harm.
If he got in with a bad crowd and started shoplifting, I'd quite likely report him, to shock him out of it. If he nicked a parking cone on a night out, I wouldn't (though I'd tell him to take it back next night!)