Pots might vary the most at the lowest volume setting but it is difficult to talk about it in terms of resistance because volume pots are used as dividers (ratio of resistance). Max difference of these ratios is often specified by pot manufacturer - often up to 3dB for good pots.
Stereo Volume Pot
Anyone knows what's the normal difference between channels when measured by DMM at any particular volume setting if pot is let's say 250K?