one way to make sure the dome light is off is to get in, and arm the alarm (but wait about 10 seconds after the alarm is armed because there is a delay for the dome light) or just get in, close the door (or do the penny trick) and press the dome light lens once, this will turn it off

manuals always say to disconnect the battery to be on the safe side, the only things ive really disconnected the battery for so far is replacing my fuel system, if im going to be reaching down towards the fan and when installing audio equip (and thats usually just cause i know im gonna have to disconnect it anyway to run power to the amp) better safe than sorry.. but for replacing bulbs i dont really bother