When you think your rotors/drums are 'warped', here's what's usually happening. Pad material is burning itself into the surface unevenly. So some areas are more 'slippery' than others. I bet that can happen with drums just as easily as disks. It's possible for them to be physically warped, but pad transfer seems to be more common.

Other things to check, so you can rule them out...

Bent rim?
Bent hub?
Damaged tires?