I like to wait until I have a bit of traffic on my sites. (at least 10-15 uniques a day)
This is only for stats reasons. As if your site isn't ranked yet, you will probably get very little traffic, so if you get a click your CTR will likely be very high.
Other than that, it doesn't really make a difference. Some people say your site will rank faster without any ads, but I don't really see the difference, with or without.