For the practical test, there are things you must memorize, and things you can look up. During the oral, you don’t have to answer from memory anything you’d have time to look up in reality. You never need to memorize and know everything. Categorize material as:
a. Things you must memorize (i.e. emergency procedures, radio calls, airspace, etc).
b. Things you must know or have reasonable understanding of (i.e. interpreting weather codes, non-critical regs).
c. Things you must know about but can look up and will have time to look up on the ground.
(Thanks to Mark Bourdeaux for this categorization.)
So if the examiner asks you about currency, it’s OK to open the FAR book to 61.56 and 61.57 and explain them to him. But make sure you know where the answer is without reading the whole FAR/AIM cover-to-cover. On the other hand, for stuff you’d have to know RIGHT NOW (e.g., best glide speed for engine failure, etc.), you’d best not stumble or stutter – know that stuff cold.
As cloud clearances were mentioned in this thread, think about that in the context of that three-point test above -- if you're flying along, will you have time to go look in the FAR/AIM to look up the answer before you get too close to the cloud?