No collars here. Why? Because many years ago when Hydrox was very young, he always wore a collar. One night, in the middle of the night, he hopped up in bed and he was wheezing terribly. I knew something was wrong, but didn't know what happened. I felt all the way around his collar and there was plenty of space the whole way around. I didn't know what else to do, so we called the vet.
First thing the vet asked was, did he have a collar on. I said yes, he always wore a collar. She told me to run my hand around the collar. I had already done that; there was nothing wrong. She told me to take the collar off. I removed his collar and he left out this huge wheeze of air. And after that he was fine.
I don't know what was wrong with his collar. We examined it very carefully and there were no threads, no loose areas, nothing to fall off. It was fine. But we never put that collar back on him again.
And since then, none of our cats has worn a collar while they were inside the house. If they go for walks outside (which is rare, since they're so much older now), they have collars and leashes. But not inside.
I've also read about cats with collars who got into closets and strangled because their collars got caught on things. I'm too paranoid.