If nothing else, dating is about honesty. There are several important reasons for this. The main element of dating is “mutual self-disclosure,” whereby each party progressively shares and reveals more about themselves.
Dating is also about being yourself and letting go, rather than anxiously screening everything you say. Marriage is based on trust, and hiding things from one’s spouse leads to an erosion of trust. Withholding significant information contradicts the basic assumption about marriage.
While every situation is unique, it is worth considering the “what” and the “when.”
You do not need to disclose every embarrassing thing that you ever did. If you feel inclined to do so, and you judge it appropriate, by all means share. But you are under no obligation to do so.
Conversely, you need to disclose major issues, such as significant financial, medical, or legal issues. You should address these issues in the context of how they are being addressed and resolved – but they should not be brushed under the carpet.
Some information is best put on the table from the outset. If this is a deal breaker, then it is best to know from the get-go. Other information is almost certainly best left until you have become closer.
The rule of thumb is that if there are really big issues, they should be disclosed as early as possible – perhaps before the dating even begins.