Depends on the amount of abuse you that get of your forum. Making people register to post is good to keep control of members, and really its so simple and obvious to do that its almost the accepted norm.
However if its working for you to have them post as any name then go for it. I think it will fall over when / if you site gets more people to it, owning to human nature.
In my Forum I am able to control who can view/post each Forum section. I can make it Public Viewing and posting--or Public viewing and Registered Members Posting.
So when I set my forum up I have the section open to the Public to View and Post. In this area I have the TOS, and the Welcomes as well as a few other BAITING items. This way a visiting guest can post an introduction, or make a comment on the things in that section...and get interested in my forum.
In the later sections I have them controlled where only registered members may post... This way I attract the interest of possible forum members by allowing them the free post here or there and get them hooked and when they want to post in the Real Disscussion areas they need to make a choice about registering and become serious.
In my experience, the early weeks/months of a forum are sane enough to skip registration. Sure, you'll get the occasional spammer, but in that phase you want to make it as easy as possible to participate.
As the forum achieves some level of popularity, though, you'll have to deal with problem posters, more spam, nickname impostors, etc. - when it hits this point, you'll have to go to registration for more tools to prevent problems.
On the plus side, by that time you'll have enough people who are forum regulars that registration won't be a big turn-off.
Similar experiance to those above, but I just thought I'd add this.
Guest posting is allowed on my forum, but the majority of posters (it's just starting out) register first. I think most people have got so used to registering to post that it probably won't make much difference whether or not you allow guest posting.