Kernel configuration for distributions

If you are going to allow such patches in to -stable releases, and backport them to older kernels, how far back do you need to go? back to the kernel the distro uses for it's release? or back before that the the kernel the distro used for the prior release (so that you can try to find problems that were introduced by the new kernel in the release, think Intel video drivers a few years ago as an example)

If it's maintained outside of the kernel.org tree, then the distro can ship the appropriate config as it goes along, and it won't matter (much) which kernel you try to compile, the make oldconfig process will work with older and newer kernels (as long as they have the feature, and the feature has not grown a dependency one some other feature, it's not perfect, but it's pretty good)