When installing fuel pump...

Depends on the material the ties are made from - I think most black-nylon types should hold up ok. I imagine some types (the cheap flimsy ones) would dissolve pretty quickly in fuel. If yours happened to dissolve, I don't think it would hurt anything, they'd just stop doing whatever you installed them to do (keep the wires separated -- which really isn't necessary as the stock install doesn't bother with anything like that) and likely drop to the bottom of the tank.

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