Everyone has a different approach to UW. I reviewed a particular topic (like biochem or respiratory), and then, once I was finished with FA and/or whatever resources I was reading from, I did all of the UW questions for that subject on untimed tutor mode. I read and answered a question, then read (not skimmed, read) the entire answer. I would often stop to look things up from my books, occasionally even before I answered the question. I would often annotate the gist of the Q&A into FA before moving on to the next question.
Because I took this approach (only doing Qs for the topic that I had reviewed that day), my UW percentage was abnormally high. I did not take that as "I will score 310 on Step 1." My goal was to use UW to get the maximum return out of the effort I put in. By doing it this way, I made sure I knew a certain topic inside-and-out before moving on to the next one. I didn't really have to go back and re-review it again after this, because I had pounded it into my head over 2-3 days.
I, personally, could not focus when doing "random" blocks - they were too all-over-the-place for any kind of concrete review session, and I found myself bouncing all over FA and my review books. I agree that it definitely simulates the exam more realistically, but, for REVIEW purposes, I found my method to better organize my train of thought. Others may prefer the random review - it doesn't really matter, as long as you see a wide variety of questions in all possible categories in your 4-6 weeks of prep.