We have long horizontal runs of cupping- curling shingles... mostly on front of the house.
Install included OSB sheathing and ELK Reflective Cool Colors comp tiles. It was a full replacement for a failing wood shingle roof. We tried to be Green.
The roofing company, which is reputable and long standing in our town, says it's just a cosmetic problem but reading on the net leads me to believe otherwise.... cupping leads to early shingle failure etc.
I've been to the attic recently to run some TV cable. I inspected the OSB. No water spots, no sponging. The only thing is that the boards seem hard butted together in all directions with absolutely no gaps whatsoever.
There is no leakage involved, the roof is watertight. The roof is under warranty, even labor.