Ignition switch off. I'll tell you a story one day years ago at job far far away I was given a carb to change out ,well heck I am good so why disconnect the battery. Well what happened is some gas spilled out ,,no big deal right ? though I did not notice the choke cable was broke and upon removing the carb the piece of cable fell down and shorted the starter battery+} to the solenoid [a Chevy starter] which caused the eletric feul pump to run which when added to the already spilled feul started burning pretty good {remember the shorted starter sparked which started the feul to catch on fire} well all I could think is how am I going to explain this to the owner of the company I worked for? But due to luck and quick thinking I pulled the neg cable from the battery which stopped the feul pump and used compressed air to blow out the fire . All this was in an instant and no damage was done . I really was lucky .So as a rule disconnect the battery and if not be carefulllllllll!!