If the cable type is the same I doubt that you'd hear a difference unless the lengths were radically different. It's not that critical because ESL's draw very little current. Ideally, they should be the same, but in the real world there are more important things to worry about.
Electrostat question
I have read many times that it is prefereable to keep the right and left speaker cables the same length for all types of speakers and I agree. Almost all high-end systems I have seen have the amplifier placed in the middle for this reason. For you ML, Quad, Sound Lab and other 'stat types, what about the AC power cable lengths? Is it just as important to have the right and left AC power cables the same length?