Hazard is my biggest concern too. However, and maybe it's a stateside thing; fearing the overreach of the "war on terror" and the effects of "if you see something, say something" thing. Stupid little things, like a college art project involving a suitcase with balloons and a sign that says "have a nice day", a silly sophomoric public art installation, which in times past would have people rolling their eyes as they walk past, now has the feds breaking down doors before supper (true story). Granted, that one does have a Keith Ledger Jokeresque vibe to it, but then so does a worldwide floatie epidemic.
Anyway, the best advise I've found today is to contact a local business that conducts or arranges mass balloon releases, and get the skinny from them, about laws and concerns. The CARs(?) in Oz and the FARs(?) in the US, these airspace regulations, are in written in bureaucrateese, and I can't deal with that. My brain ejects itself from my skull when confronted with such.