QB throwing a pass close to the LOS

[The Roamin' Umpire]

I'm in the flag first & wave it off later camp for one major reason: Chain crews (and specifically the box guy) are instructed to stay put if they see a flag on the field. (And hopefully they won't miss a flag thrown right around the LOS.) They don't have the same strong prohibition on a bean bag. I think you're more likely, with a less-than-perfect crew, to be unable to correctly determine this if you drop the bag than if you throw the flag.