If you decide to allow comments, you also have to decide whether the comments go online right away, or whether you have to approve (or moderate) the comments before hand. It's also possible to turn off comments entirely.
With the spread of other places for commentary online (like Facebook and Twitter) it's not essential that you allow comments directly, since there are plenty of places for people to type.
Blogger's comment setup guidelines are here
Wordpress on comment moderation:
Typepad has a robust set of comment settings: