Main rule for me would be to not do it alone. Someone decent watching (not playing with a phone!) Most of the time I would just go for it but if there is a big gap above a plastic tank and I am welding something rusty I will get a mate to hold a bit of sheet above the torch so any splatter goes downwards. Plastic tanks are pretty tough but big blob of molten on the top might manage to melt through. Unless it's something you are modding then also remember if it's old and rusty enough to need welding the rest of it won't be mint either. If you start pushing old fuel hoses out of the way they can split and start to seep fuel that runs along the outside of the line without you being able to see it through a mask.