There is something so empty about having zero comments on your post. Even if your blog is the best, new visitors will be turned off my zero comments. So.....if you are starting off and are still building your readership, do NOT enable your comments until you have atleast 100 RSS subscribers or 750 visitors a day, then you should switch your comments back "on" so others can see them.
I turned my comments on right away. I don't get a large number of comments even now by any means but I get a few from some dedicated readers and I found commenting on other people's blogs helps as they often return the favour.
I also enabled comments after my blog was launched but moderate it. I always delete the spam comments.