I started a thread about the conductivity of the cell case. Within that thread I linked to another forum members post whos opinion I value.
It's true the amperage between the cells negative terminal and the cells case is tiny. But what would happen if two or more of the cells cases touch each other? Installing an insulator between the cells is a good preventative measure IMO and it's cheap. Lishen specifically states in their spec sheet to not allow the "cans" to touch each other. I wouldn't rely solely on the thin PVC wrap especially in a mobile environment.
To each their own but if providing additional insulation between the cells can possibly prevent a catastrophic event then I will continue to recommend doing so.
During initial cell testing and balancing I was measuring a cell terminal voltage with my DVM. I made an assumption that my cell cases were negative and wedged one of the DVM probes under a bus bar on the negative terminal - not a problem since there is an insulating plastic piece on top of the case and even so, the case is negative anyway. The probe pivoted slightly and made contact with a small unprotected portion of the top of the case and to my surprise, melted the end off of my DVM probe. I was wrong - on these particular cells [Coslight 150 AH] the case is positive and can definitely supply significant current.
I was actually fortunate that this happened because it completely changed my initial plans for my pack design and I made provisions to eliminate any possibility of cell-to-cell case contact.