I say no. Some people just wanna sign up and browse around and see what the site content is. They may not feel like posting up just yet. I think no one should be forced to do anything. I would imagine we would prefer for people to post up only cause they browse the site and like what they see. Forcing someone to post would probably get alot of people to say eh screw it and click on the red x in the upper right hand corner.
So yeah, i say NO. 
There are a few boards i mainly lurk on, because i like what the site offers, and eventually i have made some posts, but for the most part i lurk to get what i need info wise from the boards. If i was forced to make a post, i probably would have said screw it and not bothered. :gotme: But i am pretty lazy.