^^^Pretty much what he said.
It's worth noting that even after you've taken the primary pressure off the system (released the pump pressure), the system can repressurise itself (on a small scale) just from temperature variations in the shop.
I strongly recommend that you twist the gas cap back to where it isn't sealing the tank anymore, to prevent fuel from creeping out of the open system when the vehicle is un-attended.
I've seen several instances where techs leave for the day and when they come in the next morning, their line caps are laying on the floor and there is a pool of fuel under the vehicle.
That has the potential to turn out poorly, depending on how the shop is heated. If you use torpedo style heaters...
...you're going to not even know what happened, and might wake up dead... /ubbthreads/images/graemlins/ooo.gif
Liquid fuel isn't so dangerous, it's the vapors that'll ignite. The flamable concentrations will generally occur near the floor, but may extend as high as two feet under the right/wrong circumstances.
If you leave the filler flap open, it'll remind you to tighten the cap after your fuel system service is completed.
cliffs
Take fuel seriously.
Plug the ends of the lines, clean up spills with a rag, and place that rag outside the shop to dry.
Pop the gas cap loose while the system is open to prevent the system from building pressure.