All the good quilters out there will tell you to take measurements at the top, middle, and bottom, just as the members above have instructed. However, I have NEVER been able to get accurate measurements. I can take the same measurement multiple times and get different numbers each time. I guess sometimes I stretch it as I measure and sometimes I don't. Regardless, I simply don't measure any more because I don't trust my measurements.
I calculate instead. I ASSUME that my quarter inch seam is accurate and all my squares are the intended size. for example, if my quilt blocks are 10" and I have 5 across and 7 down, then I assume my quilt top is 50x70. Add in your sashing measurements as needed, too, of course. I will cut my border strips to the measurements calculated. then I focus all my efforts on pinning. Lots of pins. to make sure anything is eased in appropriately (because, lets be honest, I likely do NOT have perfect blocks!).
Keep in mind....I have never entered a quilt show and I quilt for fun and function. so, take what I say with that in mind!
In general, practice good quilt handling. often times stretching and distortion happens without us knowing - when you pick it up and show it to your quilt friends, when you are bunching it up to push through the machine, etc etc.