This is obviously a good advice, but hacking all the machines in the lab could be quite time consuming. For me, in my early attempts, I had hack something like 12-18 servers. I still managed to get 60 points in the exam, but still, this didn't make me pass. The thing I later realize is that these servers were not picked up randomly. I was taking the approach "today, I will go after an FTP server" or "today, I am going after a web application". So I wasn't approaching a given host and try to break it, I was looking more at services...

In addition, all the exercises in the videos can be reproduce in the lab. So it's like if we are starting at 10 servers...

But on my last attempt, I felt I was ready because I targeted xxx.yyy.xxx.201, then xxx.yyy.xxx.202, then xxx.yyy.xxx.203, etc... I think I did 9 of the first 10 machines I targeted. At this point, I knew that I would eventually hack any machine I set my mind on. I then started picking up servers with very different configurations: Linux with a web server, FreeBSD with a mail server and things like that. After pwning all the machines I was targeting, I knew I was ready.

So yes, if you can, go after all machines in the lab. But if you don't have the time, you can be wise about it... 

I scheduled my exam for April 6. I was shooting for a week earlier, but to get an 8:00am Saturday time slot, I had to schedule it a week later. That gives me an extra week. I have made good progress since my last attempt and think I will be ready.