flb: I asked the same question about a year ago. The best answer I got was to check out the ledges the day before the actual hike. Since you will be staying at the Portal or somewhere close for acclimatization, you might just as well use this time to familiarize yourself with the ledges. It worked great for us and I have serious doubts that we would have found it in the dark otherwise.
Another reason why you want to be early on the MR is the danger of falling rocks caused by climbers on the East Buttress. An early start gets you into the couloir above Iceberg Lake before you have climbers overhead.
See also these threads.
Link 1 Link 2 Good luck
Kurt