They belong in the garbage.

Better yet if you go camping and build a campfire, they make great fire starters. Just put them in an empty toilet roll and toss in the fire. This keeps them out of the landfill.

The primary reason I don't use them is that they are extremely flammable, even after being washed. If you want a shock, go outside and set one on fire. It will go up like a torch, and in the blink of an eye, there will be nothing left but ash. Yes, I'm probably too security conscious, but this is just too scary for me! I use alternatives that are not so scary.

Just my opinion!

I spoke with a woman at a quilt show last year who uses used dryer sheets in her appliqued quilts, she said they are sturdy but soft enough not to change the weight or softness of the fabric and they are easy to sew through. She has been using them for years and said she has never experienced any problems.

I agree with this thought, it is no ones business. On another note, you know the lint you take out of the dryer, from your clothing, try lighting that on fire. Gone in a minute, burns very easily, too. The flammability issue is important,but the thing is cotton burns, too. No getting around that. The sensitivity issue is bad for those that suffer from it, there are those that claim sensitivity to scents but it is more a matter of not liking the scents. And that I disagree with.

I use coffee filters for applique, I find they work ok for me, then tear away. But I am a self tought sewer and there maybe a better way.

They are flammable
 

One issue is the chemicals but the biggest one is that they are flammable and melt at higher temperatures than the dryer.

I tried them awhile ago... on a doll quilt...but, I used them at least 4 times before sewing on them.
I am not currently using them, due to all the info on the board...and I agree...you can just use the cheapest interfacing you can find or I have found a stash of flannel blankets at my Aunt's house and took a stained one home.
After washing and drying I cut it up into 10 1/2" squares. I avoided the stained area and tossed that.
It looks like this was a hospital blanket, as it had the hospital name in the edging.