I watched a plane go to slow down mode because of low voltage in overhead flight toward the end of a flight. It doesn't take much off peak rpm to lose line tension, and can be an unpleasant surprise. I have "tested" batteries by by saving safe maneuvers toward the end of the flight, but that isn't a fool proof test as the load may not be as high as with overhead maneuvers. IR is not a great way to follow the cells unless you have been watching the number of that battery on your equipment to see if it changes. It can help if you find one cell significantly different. That bad cell may low volt the pack. I use a discharger with about 10 amp load and keep a cell checker on the pack to see if the cells are even under load. If that seems ok, I fly the pack safely with recoverable maneuvers toward the end. If the pack had normal capacity, even cell voltage under load, and normal IR for it, then it is likely safe.
When I data logged, I found that the average amp draw was about 20 amp for a 6S pack, and can be about double that in a hard draw maneuver, so that maneuver can discover low volt when you don't want to discover it.