Ask for OpenPGP passphrase when first encrypted chat is started.
As of version 0.9.1 of Gajim, when the OpenPGP passphrase is not save in the config file, it is asked at startup. IMHO a less intrusive solution to get it would be to ask it either when the user request an encrypted chat or (maybe better) when a message that needs this key to be decrypted is received.