A couple of rear wheel spacing thoughts...
1) Sheldon Brown comments that most 6sp axles are 137mm which will work for 130mm space frames. I don't believe that this is quite correct. Campagnolo original 6sp hubsets were often supplied with 134mm axles. 134mm axles will not work for 130mm space unless one believes that 2mm on either side of the locknuts is OK - I don't know anybody who is comfortable with this risk. Later Campy 6sp hubsets went to 136mm (not sure 137mm comes in, perhaps some 136's were a tad long but I haven't noticed that). 136mm axles leave 3mm on either side of the locknut on a 130 space frame - still not the 4.5mm - 6mm that Campy seems to gravitate towards! The unshot is that for true 130mm space frames a Campy 140mm axle is really what the patient needs.
Regarding frame squeezing issues, there is no question that running non-exact spacing results in non parallel drop outs and the risk of a drop out failure - the question, though, is what is the realistic tolerance? In reality, few builders can peg alignment to the millimeter - and the extra tweaks it takes to try to do so might perhaps be more damaging than the non parallel dropouts. Also, Campagnolo could never seem to get there hub tolerances down to a science either. Hub spacing of various configurations seems to vary by 1mm or so.
My suspicion, and others will certainly disagree, is that 2-3mm of "error" is perhaps acceptable, although 3mm would be somewhat risky. If 2mm is "acceptable", then one could could for instance have a 128mm rear and run either 126mm or 130mm space wheels. The degree of mis-alignment at the locknuts by a frame mis-spaced by 2mm is pretty negligable (but it is, of course, still there) If I'm not mistaken, I beleive that Rivendell might do something similar - as have other builders over the years. One should always inspect axles and if a bent one is discovered, it should be replaced in order to minimize the drop out failure risk.
Finally, I hope folks will realize that cold setting a frame to change the spacing rarely causes a failure, but it does kink the seatstay and chainstay. To do a re-spacing correctly some builders will actually remove the chainstay and seatstay bridges and then install or move them after the frame is cold-set. This is a tad nuts, but it can minimuze the stress in those areas. I urge people to sight down the seatstays of their frames that have been cold-set - it is not a pretty sight! Lastly, if you cold set a frame without the bridge moving treatment, my suspicion is that the operation might make a chainstay failure slightly more possible.
The upshot with all this is to pick your poison. Just some more fuel for the fires. Mike Kone