As a guild leader, your account is the lynch pin of security for your guild. You can have all the proper access controls in place on the bank to prevent even your officers from being able to rip off the bank, but nothing in the game of WoW will stop your account from making off with everything. So, every month, change your password. Even if everything is fine and you run anti-virus and you never visit porn sites, etc. Just change it and make sure it's more than eight characters long, has no words in it, no dates, and contains numbers and letters (upper and lower case).
Just some friendly neighborhood systems administrator advice.