I am a pre-washer of all yardage (not pre-cuts though). With the pre-washing comes the task of ironing and refolding the fabric. I iron the fabric flat, then I align my selvedges and wiggle them side to side so the 'bubble' at the bottom fold goes away. I then smooth from the fold to the selvedge edge then iron. That generally leaves you with uneven side edges. I trim one side to get a straight side and that is where I then do my subsequent measuring and cutting. Honestly, it probably does equate to about 2.5 inches per side in the long run.
Like Iceblossom, starting with your fabric on grain was a learned habit from garment sewing days in both home ec classes as well as from my grandmother. And while there are many quilters who do not worry so much about grain, I feel it makes your seams less likely to ravel/shred.