I did very well on step 1 (according to me, but I scored above 250 if you wanna make your own mind up about that), so here's my n=1.
I started doing UWorld questions on timed-random just after the halfway point of second year for my test date in mid to late June. Looking back, I can certainly recommend this strategy though I recognize that different strategies have worked just as well for different people. If you're going to do this, however, I will make a few points:
1) It will make you feel like **** at the beginning. No matter how many people tell you not to worry about percentage correct and to just use the questions to study, this is basically impossible to do. It's a very important test that's been 2 years in the making and UWorld bombards you with info about what percentage of people got each question right. I hadn't even had half of the second year material when I started this, so I naturally did not do well at first and it freaked me out.
2) That being said, you do learn from questions and you will start to become familiar with a lot of material before you even learn it elsewhere. UWorld is like 75% just a fancy textbook. People get too wrapped up in "saving" their review questions and ****. Just get through the questions and read the answers. Actually doing them and having time to actually utilize them well is far more important than when you do them, imho.
Just get through the questions and then reset them and do them again. Trust me, there is no paucity of places to find more questions to do for step 1.