I think the individual cell IR numbers are important. Example- I had a pack with all pretty average IR numbers of about 7.0 and within about 2-3 flights had a cell go up to over 13 and took the pack out of normal service. The entire pack's average would not have gone up that much and may not have been caught. Since I had to know how soon the pack would go, I put it in a test plane (also 10S setup) and flew 2 short easy flights before the cell in question puffed up completely and failed. Had this been in my main pattern plane, I could have lost it had it failed at a bad location in the flight. I guess the point is that your packs will probably tell you something before they fail, you just need to be watching for the signs.