I have a bank toon and a mboxing guild that has 4 mboxers in it.
I believe all 4 mboxers had full access to the guild bank, you don't put something in there that you don't want taken by someone else.

my bank toon holds all of my cash and sells/buys stuff for the team and then transfers whatever is need to the others either via guild bank or mail.
I mail everything to the bank toon and he redistributes it back out ( like cloth to the tailor, etc )
I really only use the guild bank if I need immediate transfers.

oh yeah, I have the blizzard authenticator so all of my accounts are safe... don't wow without it.