When I first started washing fabric, I did not bother to measure it before and after - until I started to sell some of the fabrics -
Even though I knew I had purchased a yard of whatever - and I usually brought home 37-38 inches of length - I thought I should verify the length before I posted it for sale. I was not very happy to find that most of my "yards" were now less than 36 inches in length.
I usually buy a bit more than the pattern lists - if it says 1/2 yard, I usually get 5/8 yard to allow for shrinkage and if it is cut crooked/off-grain.