I'm the type of person that thinks I can never be too prepared for something, but you'll have to decide whether or not it's worth the effort for you.
This!
Seriously, why wouldn't you memorize? It's timed, and every second matters. Since I memorized the conversions I had extra time to check my work, twiddle my thumbs, and look around to see that people were still working while I was waiting impatiently. The first time I took the test I didn't memorize the conversions and didn't complete all the questions.
Just because the info is supplied on the test, and the testing is for thinking on the fly doesn't mean that you shouldn't come in fully prepared. A few sayings come to mind.....
Work smarter, not harder!
If you ain't cheating you ain't trying =P
The last one is more joking of course though. Living by the first is essential in my opinion.
Like Tim said, the practice conversions aren't the same as what's on the test. I compiled a list of all volumetric, distance, area, etc. conversions. A lot of them I have never used nor heard of, but many were on the test. I assume they do this so you are less familiar with the units so they test your ability to reference and calculate. Go in with a full arsenal of weapons to destroy the test!!