Con staff have posted multiple times on their forums that they can not and will not approve weapons and props before the con - you have to bring your item to the check in area to get it approved or whatever (I'm not sure where this area is, as I've never had to bring an overly large prop with me, but I'm sure its listed on their website somewhere.)
Also, different conventions have different rules, some of which are because of past experiences at said con and some of which are because that's what the convention center or even the state says they have to do. While it would be helpful, you really can't expect every convention to have the exact same rules regarding props.
Anime Expo & Otakon
★Shizuma (Strawberry Panic!) - 50% complete
★Saber EXTRA - 85% complete
★Rachel Alucard (Blazblue) - 100% complete