K.A.
I just use regular tennis balls in the dryer. Being allergic to wool, I can't use those. The plastic ones I tried cracked open after a little while and I threw them all out, I kept one that didn't crack and I use that to keep the washer door propped open so mold doesn't grow in there when it gets hot.
They really do make a difference. I put them in with pillows after I've washed them and they come out new. They help keeps things separate so they dry better.
I don't use fabric softener or dryer sheets, these take their place along with vinegar in the washing machine.