Just going through a bit of this situation myself. Finishing up a UFO which is a 2013 BOM. None of the block fabrics are pre-washed as I don't pre-wash small pieces for fear they will fray too much and I'll not have enough. I normally pre-wash all of my yardage.
For this project, I've chosen not to pre-wash the yardage I just purchased for sashings; borders; etc. I ironed my yardage (with steam and starch) to eliminate the fold crease so I could re-fold more on-grain. I usually do not have an issue with this process and my pre-washed and ironed fabric behaves very nicely after folding/hanging to get in on-grain. This time, not so much. When cutting strips for the first set of sashing I had several strips that were bowed a bit. Not enough to be unusable but still 'off'.
My decision to not pre-wash the yardage was so that it would behave the same as the pre-cut pieces used in the blocks relative to shrinkage. Granted, that is mostly controlled by the batting used (in my case 100% cotton W&N) but still wanted all the fabrics to have the same shrinkage rate - or as close as possible given different manufacturers.
Bleeding was not my concern as I agree with most of the other posters that most fabrics do not bleed they way they used to ages ago. We'll see how my next set of sashing goes in terms of getting it on grain with this process.