Code Review: When do you do a peer code review?
A peer code review can be done prior to check-in or after check-in.

Quote:
“I find most people come down strongly on one side or another; and since we make a code review tool, this is a point of contention for a lot of our customers.”
– Jason Cohen

Application:
I advocate doing pre-commit reviews.

By doing the code reviews prior to check-in, you avoid the scenario of a developer repeating or growing an issue that could have been caught during a review process. Although a pre-commit review can create blocking/delay issues, this can also be seen as beneficial as it provides some incentive to get the reviews done quickly and avoid eternally open code reviews.

Post-commit reviews can make it slower and harder to fix bugs, because the reviews will tend to come later when the topic is not as front of mind. Also, post-commit reviews can lead to batches of code that are too big to review.

References:
http://blog.smartbear.com/software-quality/bid/170315/Code-review-before-or-after-check-in