In my opinion, there's an aspect you sould consider: if you start early you'll be overtaken, if you start late you will overtake. This has consequences on your morale.
My experience so far is:
LEL 2013, I started in the first publicly available group (B), I spent "less than five days" with a lot of cyclists overtaking me: they had started after me and were faster. I'm not a competitive cyclist, but my morale gradually worsened, and I started fearing I couldn't respect the time limit.
PBP 2015, I started in the very last group (Z) and overtook what seemed to me a lot of cyclists, I particularly remember the surprise when I first spotted a T letter, meaning somebody that had started 9 hours before me. My morale rocketed. If you think at it, it's obvious: nobody could overtake me because there was nobody behind me, I had thousands of cyclists before me, so I was simply overtaking the tail of them, but I was galvanized just the same.
Hopefully, in LEL 2017 I'll try to start in the middle just to further expand my experience...