Flagging an account means that the bank will watch it for suspicious activity. Which, in my opinion, is stupid to do. We are going to a convention where we are probably going to spend a good deal of money on food and random items we confuse ourselves into thinking we need. Most people spend a lot of money at con and if someone went and flagged their account without knowing what it would do, they probably would get shut off and then have NO way to pay for anything the rest of con.
Some people save all year to splurge at cons and flagging will do nothing but inhibit that process.
Plus, smart people who steal won't go and make large purchases at once but do a lot of smaller ones over a period of time that add up. So it would look just like a normal con spending anyways. (Only stupid thieves go splurge and get the attention of the bank.)
So really all flagging does is inhibit you if you see an expensive item you want. (Or if an emergency comes up and something bad happens. :/)
If you plan to spend a lot, a better idea is to call your bank and tell them, "Hey...I'm going to be spending a lot of money in a short period of time. Don't flag my account and don't worry about it because it's actually me." If you are worried about over the phone stuff, go to your bank and talk to them about it.
EDIT: Better yet, use cash. Girls can stick it in their bras if they are worried about losing it. Guys...ask your female friends to stick it in their bras. XP