I've reused cottons many times, never got sick or cotton fever.
However
I've also never been helped by doing so. Good quality DC brown scramble, saved cottons, always dark brown. After re-using the cotton go almost back to white, usually a light tan. Very dark solution in the water/syringe after sucking it all up to shoot, looks like a thick, good shot. Every time I've tried this happens, everything is done right, looks fine. Shot, no effect. Not even a taste, or slight lift in WDs.
In my experience re-using cottons is just as excersize in futility. It kills 20-30 minutes of sick time, and puts you at risk for a number of infections and injecting cotton fibres/cotton fever. It's a bad idea, and unless you purposely leave more dope solution on them than you would if you tried to collect each shot entirely, you're wasting your effort.
If you want to leave dope for an emergency, put a tiny bit aside from each bag. But, this never works either because this gets used before you amass enough to keep you from being sick anyway. I quit this kind of stuff early on in using. I used it all when I had it, and was sick as shit when I didn't. Being an addict you're going to be high sometimes, just well a little of the time and sick most of the time. After you accept that the highs are a little better and the lows are a little harder, but it is what it is. Used cottons of all things aren't going to help.