Hmm,
I wasn't ROTC so I can't jump in on that, plus I was an aviation guarantee so MOS selection was something I watched with mild interest but here is what I saw in my company through summer '02.
About halfway through TBS, everyone gets to fill out an MOS card listing all 22(?) MOS's from 1st to last. I think they had a provision where you couldn't put all the combat arms in the top 5 or something. Then HQMC comes down with the number of slots per MOS and your company staff alots a certain number to each third. So with say 30 infantry slots 10 top third....you get the picture. Then they take your current class standing use that to get you what you want,add SPC recommendation, slots available and finally company commander's decision.
That last one is a kicker since you may get your first choice and some other guy his 22nd. But the CO may give you your 3rd choice so the other guy can get his 5th choice.
For aviation guys its different since you have to me physically qualified and pass the ASTB. My roomate was the lucky enough to be the only guy who passed the NFO requirements in his third so he got the slot.
semper fi,
airwinger