I graduted Dec last year, got commissioned in March, worked PTAD for my OSO for 90 days or so then went to Delta. I had orders for Charlie Company (Mid April), then got my orders changed to Delta (Mid June) by the warrant officer at HQMC.
I believe it depends on your commissioning program and your contract. I think Charlie had too many aviators or something so they bumped me back to Delta since I was already on PTAD and I was PLC split. If you're PLC though, graduating in December is way better. the PLC guys get the shaft when it comes to TBS dates, so if you graduate in May, you're behind the OCC, ROTC, MECEPs, and Academy grads so you'll sit on your hands until Fox or Alpha. Some people get lucky, but if you can push to graduate a semester early, I'd recommend it. Also, if you can put in for PTAD, that was also an awesome experience, not to mention a good way to make a few bucks before you head out to The Big Suck.
Good luck,
-Itoh