Well, if having many time-travelers pre-punishing crimes is useful, then presumably having a single time-traveler pre-punish one crime is worth some fraction of that utility.
Only if (a) people know you did it, and why; and (b) you're not a one-shot time traveller, so that there is the potential for this kind of pre-punishment to happen again.
Here's an edited version of a puzzle from the book "Chuck Klosterman four" by Chuck Klosterman.
When should you punish someone for a crime they will commit in the future? Discuss.