i tend to be radically practical about such things. i almost never prewash because i prefer working with all those nasty residues. they give the fabric body, which makes it easier to work with. (AND because i'm lazy. AND because i'm fortunate in having a choice in the matter. i know many people have allergies, sensitive skin, and other issues that force them to the prewashing machine.)
it stands to reason that softener would leave a residue. i just can't picture myself worrying about that. fabric softener has been around longer than i have. i suspect that if it posed any long term danger to fabric, there'd have been a great screaming protest we'd all know about. if it ever did, it's surely been reformulated or some lawsuit would've chased it off the shelves.
given the price of fabric today, it had darned well better stand up to a little thing like fabric softener, anyway. with proper care otherwise, i'll bet a quilt well constructed today will last for generations - with otherwise proper care - whether or not it's washed with softener.
there are so many other things to "worrry" about when making a quilt. this debate could rage for ages - friendly all the way, of course. (you should read us when we get started on whether or not to prewash; the price of fabric; walmart; and one or two other subjects. we are a total HOOT!!! :lol: ) at the end of the day, it's still mostly a matter of personal preference.
if you've experienced no problems in the past using fabric softener, and plan to prewash your quilting fabric, why not put softener on some, none on others, then decide which you find easier to work with?