Absolutely not. I've seen plenty of interesting threads where they began literally three or four hours earlier, and already have hundreds of posts, most of which are non-trivial length. It's simply not practical to ask absolutely every person who wants to participate to read every single post at that point.
If the thread is still young (say, no more than 100 posts), or moving particularly slowly (e.g. it's been around a week and hasn't hit 200 posts yet), then I could see an argument for "hey, you can take the time to keep up."
I mean, consider this very thread. It now has over 5 full pages of responses, despite being less than 24 hours old. That means I'd have to read over 100 posts, many of which are multiple paragraphs, just to be allowed to participate. And this isn't even a fast-moving thread. If I had had to read every single prior post before I were allowed to post, I would just not participate. That's exactly the opposite of the point of a forum.
As others have said, if I could have more fine gradation than a binary, it would be "is thread short? then read the whole thing, that's just courteous; is thread long? read first page, and the most recent full page, to get a sense of where things are at." That's still at least 40 posts to read, but it gives you an idea of where things started and where they're at now.