Just wanted to add to this although post although it has pretty much all been said already.
A large variation in IR between cells in a pack is a bigger problem than a pack that generally has high IR for all cells (assuming as has been mentioned that a pack with high IR is not being loaded beyond its capability).
If a pack has high IR across all cells then the temperature of the pack after flight is a good indication if it is up to the job. If it is coming down cool then there should be no problems.
If a pack has a large variation in IRs then there is a chance that some cells will drop to a lower voltage after flight than the other cells. This gives rise to the possibility of over-discharging the pack even though the total pack voltage after flight is within acceptable limits and the mAh put back in the pack after charge is also within the 80% rule. In this situation you need to determine flight time etc based on the performance of the weakest cell in the pack not the entire pack.
The difference in cell voltage after charging is not really going to tell you a lot (especially if using a decent charger) as the pack will generally balance and its not going to change much without a load on it unless its left for a reasonable length of time.
The IR reading on the charger may not be accurate but assuming the measurement is consistent across cells then it should reveal certain trends.
As an example I have a 6S nanotech which has been rubbish and started puffing after about 15 cycles. Its showing two cells with an IR approx 50% greater than the other 2 (using a cellpro 10S). Just today I noticed that even though all cells were balanced before flight the battery came off the heli with a cell at around 3.4V whilst the other cells are at around 3.7-3.8. IMO this pack is not going to last much longer and im going to have to drop the flight time on this pack (if I use it again) to prevent the low voltage cell dropping below the 3 volt limit.