I think it depends on the load on the cables.  I have 3 S9's from different batches.  The ones that burned (the most) are the ones that are running at the highest freq and therefor pulling the most juice.  It seems like for various batches bitmain would mix two high performing boards with a lower performing board and I can tell just from looking at the connectors which is which.  This is especially true on the autotune ones where different boards set different freqs.

I think the original EVGA cables were not built for the sustained load these things put on them.

Since I tried the conductive grease and the grease has darkened my connectors even more, I go entirely on how hot they are to the touch.  If it is too hot to keep my fingers on the cables and connectors indefinitely, then it's too hot in my opinion.  Right now I can put my fingers on any of the connectors or the cables near the connectors and hold them there as long as I want without feeling like it is getting too hot to keep touching it.  But again, I replaced the original EVGA cables for all of the ones that are pulling the highest loads.