I guess I am going to be the weird one and say I take the actual fabric with me (I have been known to wander the store with a large stuffed 2 gallon ziplock bag holding it up to fabric bolts--I have taken either individual fabrics I want to match with or even a collection of 8 fabrics I want to add one more to).
I don't really want to cut into my fabric when I am not sure if I will have enough (especially if you lose several inches getting it straight of grain--lost well over 6 inches due to shrinkage and straightening just one yard a few weeks ago!!).