I have found this to be a good practice after finishing an Iteration. The code is fresh in your mind and you can fix the bugs faster than if you wait until a couple weeks later. Fixing those bugs should be included in your iteration planning.

Back in 2000 Joel Spolsky wrote his well known 12 Steps to Better Code article and he makes some interesting comments regarding having your code ready to ship at all time. (http://www.joelonsoftware.com/articles/fog0000000043.html)