It’s safe until it’s not. I worked at gas stations throughout high school and college and saw a few gas spills caused by the auto-shutoff system not activating. In all cases the amount spilled was minimized by the pumper being able to turn it off quickly (even with the built in trigger holder engaged. A zip tie would take a bit longer, but you’d probably still be fine. Jamming the gas cap in would also be quick to remove. I’ve seen rubber things sold for this purpose, too.