I don't know how it works at other schools, but at mine, filling out your preference/bid card, signing it and turning it in counts as accepting a bid. There was nothing to sign after you got your bid. Pretty new Greek system (celebrating 10 years this year) with no-frills, release figures, etc. from the start, so I'm guessing this is the way NPC wants it done now...but I could be wrong.
Another thing - Panhellenic and/or recruitment counselors should be explaining all of this, very clearly, to the PNMs when they're in that room at the end, filling out their bid cards. If you maximize your choices on your card - say 1,2,3 - get your #3 choice, then decide to drop, you will not be eligible to accept membership in any NPC organization for one year.