it depends... if you are going to use any fusable product or starch you should not add fabric softener.
when prewashing i never add fabric softener, i do add it when i wash the finished quilt. but never when prewashing, in fact when i prewash i run the fabric through the cycles twice, first with a small amount of detergent the second time without so i know there is nothing left in the fabric that may cause a problem no matter what i'm going to do...be it using a fusable or dyeing the fabric.