If you want it to correct for the alcohol level in the wort once fermentation is underway you will need to. I sometimes take a couple of quick FG checks with refractometer over the course of a couple of days to make sure fermentation has completed. If you don't want to do the math in your head it's much easier to punch the number into software. The first few times I used it for FG I kept thinking, "Oh, no, this didnt' finish out!! Why is the number so high?!?!" Took me a couple of seconds to remember the alcohol was having an effect on my reading.