There are no rules, just find what does the job.

I remember once at a festival there was a group from Madagascar or somewhere. They'd got a kind of washtub bass, but with a more musical tone. The "string" they used was a weird wiggly metal thing they'd got off a packing case. They said that when they got some bookings they went out and bought a proper bass string from a music shop, but it just didn't make the right sound, so they wen't back to the binding wire, and it sounded fine.