I guess one should take precaution and judge the person before taking any such step because high emotions are attached to any such steps and many times people cant just take step in right direction when they are going through high emotions.
If its just a past relationship and you think that your partner can take it, you should tell even before getting married and if you think otherwise, take your time.
Well, we did break up but that was over a very small issue.....but you know with women, when they're angry they concentrate on all the negatives.
I think telling everything before marriage is a bad idea for both man and woman but if you fear that somebody else would tell your secrets to your spouse then it's better to share those things.